minor #17100 [Mailer] Document the stop() method of SmtpTransport (94noni)

This PR was merged into the 6.1 branch.

Discussion
----------

[Mailer] Document the stop() method of SmtpTransport

Ref #16473
Also see the comment symfony/symfony#45307 (comment)
The text is heavily inspired by the code comment

Commits
-------

18006fb [Mailer] Document the stop() method of SmtpTransport

Author: javiereguiluz

mailer.rst

@@ -1240,6 +1240,19 @@ disable asynchronous delivery.
                 ->messageBus('app.another_bus');
         };
 
+.. note::
+
+    In cases of long-running scripts, and when Mailer uses the
+    :class:`Symfony\\Component\\Mailer\\Transport\\Smtp\\SmtpTransport`
+    you may manually disconnect from the SMTP server to avoid keeping
+    an open connection to the SMTP server in between sending emails.
+    You can do so by using the ``stop()`` method.
+
+.. versionadded:: 6.1
+
+    The :method:`Symfony\\Component\\Mailer\\Transport\\Smtp\\SmtpTransport::stop()`
+    method was made public in Symfony 6.1.
+
 Adding Tags and Metadata to Emails
 ----------------------------------