minor #13707 Add a note about allow_no_handlers option (Nek-)

This PR was submitted for the 5.0 branch but it was merged into the 4.4 branch instead.

Discussion
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Add a note about allow_no_handlers option

This option was not documented at this state.

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Commits
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8f24431 Add a note about allow_no_handlers option

Author: javiereguiluz

messenger/multiple_buses.rst

@@ -105,6 +105,11 @@ This will create three new services:
 
 * ``event.bus``: autowireable with ``MessageBusInterface $eventBus``.
 
+.. note::
+
+    The ``allow_no_handlers`` option allows ``event.bus`` to have no handler configured
+    and as a result, it will not end up with error if you define no handler for this bus.
+
 Restrict Handlers per Bus
 -------------------------