[Messenger] Register a handler for a specific bus

Author: dunglas

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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~