[Mailer] add more info for debugging

Author: 94noni

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