Improve description of channels and channel types (nextflow-io#4120)

Signed-off-by: Ben Sherman <[email protected]>

docs/channel.md

@@ -2,12 +2,12 @@
 
 # Channels
 
-Nextflow is based on the Dataflow programming model in which processes communicate through channels.
+Nextflow is based on the dataflow programming model in which processes communicate through channels.
 
 A channel has two major properties:
 
-1. Sending a message is an *asynchronous* operation which completes immediately, without having to wait for the receiving process.
-2. Receiving data is a blocking operation which stops the receiving process until the message has arrived.
+1. Sending a message is an *asynchronous* (i.e. non-blocking) operation, which means the sender doesn't have to wait for the receiving process.
+2. Receiving a message is a *synchronous* (i.e. blocking) operation, which means the receiving process must wait until a message has arrived.
 
 (channel-types)=
 
@@ -27,12 +27,10 @@ A queue channel can be created by factory methods ([of](#of), [fromPath](#frompa
 
 ### Value channel
 
-A *value channel* a.k.a. *singleton channel* is bound to a single value and can be read any number of times without being consumed.
+A *value channel* contains a single value and can be consumed any number of times by a process or operator.
 
 A value channel can be created with the [value](#value) factory method or by any operator that produces a single value ({ref}`operator-first`, {ref}`operator-collect`, {ref}`operator-reduce`, etc). Additionally, a process will emit value channels if it is invoked with all value channels, including simple values which are implicitly wrapped in a value channel.
 
-A value channel is implicitly created by a process when it is invoked with a simple value. Furthermore, a value channel is also implicitly created as output for a process whose inputs are all value channels.
-
 For example:
 
 ```groovy
@@ -54,7 +52,7 @@ workflow {
 }
 ```
 
-In the above example, since the `foo` process is invoked with a simple value instead of a channel, the input is implicitly converted to a value channel, and the output is also emitted as a value channel.
+In the above example, since the `foo` process is invoked with a simple value instead of a channel, the input is implicitly wrapped in a value channel, and the output is also emitted as a value channel.
 
 See also: {ref}`process-multiple-input-channels`.