minor #6452 Use different placeholders in mailer config (sblaut)

This PR was submitted for the 3.0 branch but it was merged into the 2.3 branch instead (closes #6452).

Discussion
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Use different placeholders in mailer config

The placeholders 'AWS_ACCESS_KEY' and 'AWS_SECRET_KEY' are misleading, given that the AWS documentation explicitly states:
"Your SMTP user name and password are not the same as your AWS access key ID and secret access key. Do not attempt to use your AWS credentials to authenticate yourself to the Amazon SES SMTP endpoint."
Source: http://docs.aws.amazon.com/ses/latest/DeveloperGuide/get-smtp-credentials.html

Commits
-------

ba047f1 Use different placeholders in mailer config

cookbook/email/cloud.rst

@@ -36,8 +36,8 @@ and complete the configuration with the provided ``username`` and ``password``:
             host:       email-smtp.us-east-1.amazonaws.com
             port:       465 # different ports are available, see SES console
             encryption: tls # TLS encryption is required
-            username:   AWS_ACCESS_KEY  # to be created in the SES console
-            password:   AWS_SECRET_KEY  # to be created in the SES console
+            username:   AWS_SES_SMTP_USERNAME  # to be created in the SES console
+            password:   AWS_SES_SMTP_PASSWORD  # to be created in the SES console
 
     .. code-block:: xml
 
@@ -57,8 +57,8 @@ and complete the configuration with the provided ``username`` and ``password``:
                 host="email-smtp.us-east-1.amazonaws.com"
                 port="465"
                 encryption="tls"
-                username="AWS_ACCESS_KEY"
-                password="AWS_SECRET_KEY"
+                username="AWS_SES_SMTP_USERNAME"
+                password="AWS_SES_SMTP_PASSWORD"
             />
         </container>
 
@@ -70,8 +70,8 @@ and complete the configuration with the provided ``username`` and ``password``:
             'host'       => 'email-smtp.us-east-1.amazonaws.com',
             'port'       => 465,
             'encryption' => 'tls',
-            'username'   => 'AWS_ACCESS_KEY',
-            'password'   => 'AWS_SECRET_KEY',
+            'username'   => 'AWS_SES_SMTP_USERNAME',
+            'password'   => 'AWS_SES_SMTP_PASSWORD',
         ));
 
 The ``port`` and ``encryption`` keys are not present in the Symfony Standard
@@ -96,8 +96,8 @@ And that's it, you're ready to start sending emails through the cloud!
             mailer_host:       email-smtp.us-east-1.amazonaws.com
             mailer_port:       465 # different ports are available, see SES console
             mailer_encryption: tls # TLS encryption is required
-            mailer_user:       AWS_ACCESS_KEY # to be created in the SES console
-            mailer_password:   AWS_SECRET_KEY # to be created in the SES console
+            mailer_user:       AWS_SES_SMTP_USERNAME # to be created in the SES console
+            mailer_password:   AWS_SES_SMTP_PASSWORD # to be created in the SES console
 
 .. note::