minor #14430 [Messenger] Routing & Inheritance (sylfabre)

This PR was submitted for the 5.x branch but it was squashed and merged into the 4.4 branch instead.

Discussion
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[Messenger] Routing & Inheritance

This PR details how Messenger deals with messages and inheritance.

We were discussing with my colleagues and had to do a quick POC to find out as it is not documented, and we had some doubt about it.

Commits
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f5ee472 [Messenger] Routing & Inheritance

Author: wouterj

messenger.rst

@@ -314,6 +314,13 @@ to multiple transports:
             ],
         ]);
 
+.. note::
+
+    If you configure routing for both a child and parent class, but rules
+    are used. E.g. if you have an ``SmsNotification`` object that extends
+    from ``Notification``, both the routing for ``Notification`` and
+    ``SmsNotification`` will be used.
+
 Doctrine Entities in Messages
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~