minor #18848 [Mailer][Messenger] Mention that emails are async when using Messenger (javiereguiluz)

This PR was submitted for the 6.4 branch but it was merged into the 5.4 branch instead.

Discussion
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[Mailer][Messenger] Mention that emails are async when using Messenger

Fixes #17072.

Commits
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585b8a3 [Mailer] Mention that emails are async when using Messenger

Author: javiereguiluz

mailer.rst

@@ -398,9 +398,12 @@ and create an :class:`Symfony\\Component\\Mime\\Email` object::
         }
     }
 
-That's it! The message will be sent via the transport you configured. If the
-transport is configured to :ref:`send emails asynchronously <mailer-sending-messages-async>`,
-the message won't be actually sent until :doc:`a worker consumes it <messenger-worker>`.
+That's it! The message will be sent immediately via the transport you configured.
+If you prefer to send emails asynchronously to improve performance, read the
+:ref:`Sending Messages Async <mailer-sending-messages-async>` section. Also, if
+your application has the :doc:`Messenger component </messenger>` installed, all
+emails will be sent asynchronously by default
+(but :ref:`you can change that <messenger-handling-messages-synchronously>`).
 
 Email Addresses
 ~~~~~~~~~~~~~~~

messenger.rst

@@ -391,6 +391,8 @@ Then, in your handler, you can query for a fresh object::
 
 This guarantees the entity contains fresh data.
 
+.. _messenger-handling-messages-synchronously:
+
 Handling Messages Synchronously
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~