minor #17340 [Mailer] docs: add tip about listener priority to mailer.rst (rogamoore)

This PR was merged into the 4.4 branch.

Discussion
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[Mailer] docs: add tip about listener priority to mailer.rst

When signing or encrypting a message in a listener, it's important that the body has already been rendered - otherwise the signature will be invalid. As I spent a few hours to realize this, I suggest to add the attached tip to the documentation.

See symfony/symfony#39354 (comment) for another example.

Commits
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19570b8 add tip about listener priority to mailer.rst

Author: javiereguiluz

mailer.rst

@@ -1057,6 +1057,14 @@ is sent::
         }
     }
 
+.. tip::
+
+    The ``MessageEvent`` is also used internally. Depending on your use case
+    you might need to set a lower or higher priority for your own listener.
+    For example, when you want to sign the message, make sure to use ``-1``
+    or lower so the body has already been rendered and will not change after
+    signing.
+
 Development & Debugging
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