Align docs etc with new discovery setting names

In elastic#38333 and elastic#38350 we moved away from the `discovery.zen` settings namespace
since these settings have an effect even though Zen Discovery itself is being
phased out. This change aligns the documentation and the names of related
classes and methods with the newly-introduced naming conventions.

docs/plugins/discovery-azure-classic.asciidoc

@@ -1,7 +1,8 @@
 [[discovery-azure-classic]]
 === Azure Classic Discovery Plugin
 
-The Azure Classic Discovery plugin uses the Azure Classic API for unicast discovery.
+The Azure Classic Discovery plugin uses the Azure Classic API to identify the
+addresses of seed hosts.
 
 // TODO: Link to ARM plugin when ready
 // See issue https://github.com/elastic/elasticsearch/issues/19146
@@ -14,8 +15,8 @@ include::install_remove.asciidoc[]
 [[discovery-azure-classic-usage]]
 ==== Azure Virtual Machine Discovery
 
-Azure VM discovery allows to use the azure APIs to perform automatic discovery (similar to multicast in non hostile
-multicast environments). Here is a simple sample configuration:
+Azure VM discovery allows to use the azure APIs to perform automatic discovery.
+Here is a simple sample configuration:
 
 [source,yaml]
 ----

docs/plugins/discovery-ec2.asciidoc

@@ -1,7 +1,8 @@
 [[discovery-ec2]]
 === EC2 Discovery Plugin
 
-The EC2 discovery plugin uses the https://github.com/aws/aws-sdk-java[AWS API] for unicast discovery.
+The EC2 discovery plugin uses the https://github.com/aws/aws-sdk-java[AWS API]
+to identify the addresses of seed hosts.
 
 *If you are looking for a hosted solution of Elasticsearch on AWS, please visit http://www.elastic.co/cloud.*
 
@@ -11,12 +12,12 @@ include::install_remove.asciidoc[]
 [[discovery-ec2-usage]]
 ==== Getting started with AWS
 
-The plugin provides a hosts provider for zen discovery named `ec2`. This hosts
-provider finds other Elasticsearch instances in EC2 through AWS metadata.
-Authentication is done using
+The plugin adds a seed hosts provider named `ec2`. This seed hosts provider
+finds other Elasticsearch instances in EC2 by querying the AWS metadata
+service.  Authentication is done using
 http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html[IAM
-Role] credentials by default. To enable the plugin, set the unicast host
-provider for Zen discovery to `ec2`:
+Role] credentials by default. To enable the plugin, configure {es} to use the
+`ec2` seed hosts provider:
 
 [source,yaml]
 ----
@@ -25,9 +26,9 @@ discovery.seed_providers: ec2
 
 ==== Settings
 
-EC2 host discovery supports a number of settings.  
-Some settings are sensitive and must be stored in the {ref}/secure-settings.html[elasticsearch keystore].
-For example, to use explicit AWS access keys:
+EC2 discovery supports a number of settings.  Some settings are sensitive and
+must be stored in the {ref}/secure-settings.html[elasticsearch keystore].  For
+example, to use explicit AWS access keys:
 
 [source,sh]
 ----
@@ -184,15 +185,18 @@ Management Console. It should look similar to this.
 [[discovery-ec2-filtering]]
 ===== Filtering by Tags
 
-The ec2 discovery can also filter machines to include in the cluster based on tags (and not just groups). The settings
-to use include the `discovery.ec2.tag.` prefix. For example, if you defined a tag `stage` in EC2 and set it to `dev`,
-setting `discovery.ec2.tag.stage` to `dev` will only filter instances with a tag key set to `stage`, and a value
-of `dev`. Adding multiple `discovery.ec2.tag` settings will require all of those tags to be set for the instance to be included.
+The ec2 discovery plugin can also filter machines to include in the cluster
+based on tags (and not just groups). The settings to use include the
+`discovery.ec2.tag.` prefix. For example, if you defined a tag `stage` in EC2
+and set it to `dev`, setting `discovery.ec2.tag.stage` to `dev` will only
+filter instances with a tag key set to `stage`, and a value of `dev`. Adding
+multiple `discovery.ec2.tag` settings will require all of those tags to be set
+for the instance to be included.
 
-One practical use for tag filtering is when an ec2 cluster contains many nodes that are not running Elasticsearch. In
-this case (particularly with high `discovery.zen.ping_timeout` values) there is a risk that a new node's discovery phase
-will end before it has found the cluster (which will result in it declaring itself master of a new cluster with the same
-name - highly undesirable). Tagging Elasticsearch ec2 nodes and then filtering by that tag will resolve this issue.
+One practical use for tag filtering is when an ec2 cluster contains many nodes
+that are not master-eligible {es} nodes. In this case, tagging the ec2
+instances that _are_ running the master-eligible {es} nodes, and then filtering
+by that tag, will help discovery to run more efficiently.
 
 [[discovery-ec2-attributes]]
 ===== Automatic Node Attributes
@@ -229,7 +233,6 @@ Prefer https://aws.amazon.com/amazon-linux-ami/[Amazon Linux AMIs] as since Elas
 
 ===== Networking
 * Networking throttling takes place on smaller instance types in both the form of https://lab.getbase.com/how-we-discovered-limitations-on-the-aws-tcp-stack/[bandwidth and number of connections]. Therefore if large number of connections are needed and networking is becoming a bottleneck, avoid https://aws.amazon.com/ec2/instance-types/[instance types] with networking labeled as `Moderate` or `Low`.
-* Multicast is not supported, even when in an VPC; the aws cloud plugin which joins by performing a security group lookup.
 * When running in multiple http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-regions-availability-zones.html[availability zones] be sure to leverage {ref}/allocation-awareness.html[shard allocation awareness] so that not all copies of shard data reside in the same availability zone.
 * Do not span a cluster across regions.  If necessary, use a cross cluster search.
 

docs/plugins/discovery-gce.asciidoc

@@ -1,16 +1,17 @@
 [[discovery-gce]]
 === GCE Discovery Plugin
 
-The Google Compute Engine Discovery plugin uses the GCE API for unicast discovery.
+The Google Compute Engine Discovery plugin uses the GCE API to identify the
+addresses of seed hosts.
 
 :plugin_name: discovery-gce
 include::install_remove.asciidoc[]
 
 [[discovery-gce-usage]]
 ==== GCE Virtual Machine Discovery
 
-Google Compute Engine VM discovery allows to use the google APIs to perform automatic discovery (similar to multicast
-in non hostile multicast environments). Here is a simple sample configuration:
+Google Compute Engine VM discovery allows to use the google APIs to perform
+automatic discovery of seed hosts. Here is a simple sample configuration:
 
 [source,yaml]
 --------------------------------------------------
@@ -358,11 +359,10 @@ The GCE discovery can also filter machines to include in the cluster based on ta
 For example, setting `discovery.gce.tags` to `dev` will only filter instances having a tag set to `dev`. Several tags
 set will require all of those tags to be set for the instance to be included.
 
-One practical use for tag filtering is when an GCE cluster contains many nodes that are not running
-Elasticsearch. In this case (particularly with high `discovery.zen.ping_timeout` values) there is a risk that a new
-node's discovery phase will end before it has found the cluster (which will result in it declaring itself master of a
-new cluster with the same name - highly undesirable). Adding tag on Elasticsearch GCE nodes and then filtering by that
-tag will resolve this issue.
+One practical use for tag filtering is when a GCE cluster contains many nodes
+that are not master-eligible {es} nodes. In this case, tagging the GCE
+instances that _are_ running the master-eligible {es} nodes, and then filtering
+by that tag, will help discovery to run more efficiently.
 
 Add your tag when building the new instance:
 

docs/plugins/discovery.asciidoc

@@ -1,8 +1,9 @@
 [[discovery]]
 == Discovery Plugins
 
-Discovery plugins extend Elasticsearch by adding new hosts providers that can be
-used to extend the {ref}/modules-discovery.html[cluster formation module].
+Discovery plugins extend Elasticsearch by adding new seed hosts providers that
+can be used to extend the {ref}/modules-discovery.html[cluster formation
+module].
 
 [float]
 ==== Core discovery plugins
@@ -12,22 +13,22 @@ The core discovery plugins are:
 <<discovery-ec2,EC2 discovery>>::
 
 The EC2 discovery plugin uses the https://github.com/aws/aws-sdk-java[AWS API]
-for unicast discovery.
+to identify the addresses of seed hosts.
 
 <<discovery-azure-classic,Azure Classic discovery>>::
 
-The Azure Classic discovery plugin uses the Azure Classic API for unicast
-discovery.
+The Azure Classic discovery plugin uses the Azure Classic API to identify the
+addresses of seed hosts.
 
 <<discovery-gce,GCE discovery>>::
 
-The Google Compute Engine discovery plugin uses the GCE API for unicast
-discovery.
+The Google Compute Engine discovery plugin uses the GCE API to identify the
+addresses of seed hosts.
 
 [float]
 ==== Community contributed discovery plugins
 
-A number of discovery plugins have been contributed by our community:
+The following discovery plugins have been contributed by our community:
 
 * https://github.com/fabric8io/elasticsearch-cloud-kubernetes[Kubernetes Discovery Plugin] (by Jimmi Dyson, http://fabric8.io[fabric8])
 

docs/reference/migration/migrate_7_0/plugins.asciidoc

@@ -41,7 +41,7 @@ renamed to `requiresAnalysisSettings`
 * This plugin has been removed since its functionality is now part of
 Elasticsearch and requires no plugin. The location of the hosts file has moved
 from `$ES_PATH_CONF/file-discovery/unicast_hosts.txt` to
-`$ES_PATH_CONF/unicast_hosts.txt`. See <<file-based-hosts-provider, the
+`$ES_PATH_CONF/unicast_hosts.txt`. See <<file-based-hosts-provider,the
 file-based hosts provider documentation>> for further information.
 
 [float]

docs/reference/modules/discovery/discovery-settings.asciidoc

@@ -130,9 +130,30 @@ Discovery and cluster formation are affected by the following settings:
     considering the request to have failed. Defaults to `3s`.    
 
 `discovery.seed_providers`::
-    Specifies which types of <<built-in-hosts-providers,seed provider>> provide
-    the addresses of seed nodes. By default, it is the
-    <<settings-based-hosts-provider,settings-based hosts provider>>.
+    Specifies which types of <<built-in-hosts-providers,seed hosts provider>>
+    to use to obtain the addresses of the seed nodes used to start the
+    discovery process. By default, it is the
+    <<settings-based-hosts-provider,settings-based seed hosts provider>>.
+    This setting was previously known as `discovery.zen.hosts_provider`. Its
+    old name is deprecated but continues to work in order to preserve backwards
+    compatibility. Support for the old name will be removed in a future
+    version.
+
+`discovery.seed_resolver.max_concurrent_resolvers`::
+    Specifies how many concurrent DNS lookups to perform when resolving the
+    addresses of seed nodes. Defaults to `10`. This setting was previously
+    known as `discovery.zen.ping.unicast.concurrent_connects`. Its old name is
+    deprecated but continues to work in order to preserve backwards
+    compatibility. Support for the old name will be removed in a future
+    version.
+
+`discovery.seed_resolver.timeout`::
+    Specifies how long to wait for each DNS lookup performed when resolving the
+    addresses of seed nodes. Defaults to `5s`. This setting was previously
+    known as `discovery.zen.ping.unicast.hosts.resolve_timeout`. Its old name
+    is deprecated but continues to work in order to preserve backwards
+    compatibility. Support for the old name will be removed in a future
+    version.
 
 [[no-master-block]]`cluster.no_master_block`::
 Specifies which operations are rejected when there is no active master in a
@@ -169,6 +190,10 @@ earlier versions. The `discovery.zen.no_master_block` setting is ignored.
     format `host:port` or `host`, where `port` defaults to the setting
     `transport.profiles.default.port`. Note that IPv6 hosts must be bracketed.
     The default value is `127.0.0.1, [::1]`. See <<unicast.hosts>>.
+    This setting was previously known as `discovery.zen.ping.unicast.hosts`.
+    Its old name is deprecated but continues to work in order to preserve
+    backwards compatibility. Support for the old name will be removed in a
+    future version.
 
 `discovery.seed_resolver.timeout`::