minor #17950 [Mailer] add more info for debugging (94noni)

This PR was submitted for the 6.2 branch but it was squashed and merged into the 5.4 branch instead.

Discussion
----------

[Mailer] add more info for debugging

Hi

I did not understand this section of getDebug until I navigate the code and see the swap to do with
- MailerInterface and TransportInterface

`MailerInterface`

$a = $this->mailer->send($email); // $a is null

`TransportInterface`

$a = $this->mailer->send($email); // $a is SentMessage

Commits
-------

82606fe [Mailer] add more info for debugging

Author: javiereguiluz

mailer.rst

@@ -663,6 +663,12 @@ provides access to the original message (``getOriginalMessage()``) and to some
 debug information (``getDebug()``) such as the HTTP calls done by the HTTP
 transports, which is useful to debug errors.
 
+.. note::
+
+    You will need to replace :class:`Symfony\\Component\\Mailer\\MailerInterface`
+    with :class:`Symfony\\Component\\Mailer\\Transport\\TransportInterface` to have
+    the message object returned.
+
 .. note::
 
     Some mailer providers change the ``Message-Id`` when sending the email. The