minor #20164 [Mailer] Update mailer.rst (GrinWay)

This PR was submitted for the 7.1 branch but it was squashed and merged into the 6.4 branch instead.

Discussion
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[Mailer] Update mailer.rst

This piece of addition information will solve the problem of misunderstanding: Why my emails can't be processed asynchronously.

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Commits
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9c2d1d9 [Mailer] Update mailer.rst

Author: javiereguiluz

mailer.rst

@@ -834,6 +834,13 @@ The exceptions related to mailer transports (those which implement
 :class:`Symfony\\Component\\Mailer\\Exception\\TransportException`) also provide
 this debug information via the ``getDebug()`` method.
 
+But you have to keep in mind that using :class:`Symfony\\Component\\Mailer\\Transport\\TransportInterface`
+you can't rely on asynchronous sending emails.
+It doesn't use a bus to dispatch :class:`Symfony\\Component\\Mailer\\Messenger\\SendEmailMessage`.
+
+Use :class:`Symfony\\Component\\Mailer\\MailerInterface` if you want to have an opportunity
+to send emails asynchronously.
+
 .. _mailer-twig:
 
 Twig: HTML & CSS