minor #10487 [Messenger] Register a handler for a specific bus (dunglas)

This PR was squashed before being merged into the 4.1 branch (closes #10487).

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[Messenger] Register a handler for a specific bus

This is currently not documented.

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6ed93b2 [Messenger] Register a handler for a specific bus

Author: javiereguiluz

messenger.rst

@@ -449,6 +449,12 @@ this:
 This will generate the ``messenger.bus.commands`` and ``messenger.bus.events`` services
 that you can inject in your services.
 
+.. note::
+
+    To register a handler for only a specific bus, add a ``bus`` attribute having the
+    name of this bus as value to the ``messenger.message_handler`` tag in the handler's
+    service definition.
+
 Type-hints and Auto-wiring
 ~~~~~~~~~~~~~~~~~~~~~~~~~~