Update mailer.rst

Author: alexander-schranz

mailer.rst

@@ -1027,7 +1027,7 @@ Events
 MessageEvent
 ~~~~~~~~~~~~
 
-The MessageEvent allows you to manipulate the send message::
+The MessageEvent allows the transformation of a Message and the Envelope before the email is sent:
 
     use Symfony\Component\EventDispatcher\EventSubscriberInterface;
     use Symfony\Component\Mailer\Event\MessageEvent;