Add a note about allow_no_handlers option

This option was not documented at this state.

Author: Nek-

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 and as a result,
+    it will not fail if you defines no handler for this bus.
+
 Restrict Handlers per Bus
 -------------------------