minor #12765 Explain the prefix used for expressions in YAML files (javiereguiluz)

This PR was merged into the 3.4 branch.

Discussion
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Explain the prefix used for expressions in YAML files

The `@=` prefix is shown but never explained, so let's add a comment about it.

Commits
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6ed4809 Explain the prefix used for expressions in YAML files

Author: javiereguiluz

service_container/expression_language.rst

@@ -28,6 +28,7 @@ to another service: ``AppBundle\Mailer``. One way to do this is with an expressi
             AppBundle\Mail\MailerConfiguration: ~
 
             AppBundle\Mailer:
+                # the '@=' prefix is required when using expressions for arguments in YAML files
                 arguments: ["@=service('AppBundle\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]
 
     .. code-block:: xml
@@ -80,6 +81,7 @@ via a ``container`` variable. Here's another example:
 
         services:
             AppBundle\Mailer:
+                # the '@=' prefix is required when using expressions for arguments in YAML files
                 arguments: ["@=container.hasParameter('some_param') ? parameter('some_param') : 'default_value'"]
 
     .. code-block:: xml