minor #17998 Update mailer.rst (orangevinz)

This PR was merged into the 5.4 branch.

Discussion
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Update mailer.rst

It's not clear to me that `$this->assertEmailCount(1);` returns `0` when _Messenger_ is used
and does not concern queued messages.

Commits
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58979f6 Update mailer.rst

Author: OskarStark

mailer.rst

@@ -1582,3 +1582,8 @@ the :class:`Symfony\\Bundle\\FrameworkBundle\\Test\\MailerAssertionsTrait`::
 .. _`default_socket_timeout`: https://www.php.net/manual/en/filesystem.configuration.php#ini.default-socket-timeout
 .. _`RFC 3986`: https://www.ietf.org/rfc/rfc3986.txt
 .. _`App Password`: https://support.google.com/accounts/answer/185833
+
+.. tip::
+
+    If you're using Messenger you should use ``$this->assertQueuedEmailCount(1);`` instead.
+