[EventDispatcher] Document dispatch() with one argument as a FQCN

components/event_dispatcher.rst

@@ -348,6 +348,8 @@ of the event to dispatch::
     // creates the OrderPlacedEvent and dispatches it
     $event = new OrderPlacedEvent($order);
     $dispatcher->dispatch($event, OrderPlacedEvent::NAME);
+    // note that the second argument ``OrderPlacedEvent::NAME`` is optional,
+    // read more below in the subscriber code part
 
 Notice that the special ``OrderPlacedEvent`` object is created and passed to
 the ``dispatch()`` method. Now, any listener to the ``order.placed``
@@ -391,6 +393,10 @@ Take the following example of a subscriber that subscribes to the
                     ['onKernelResponsePost', -10],
                 ],
                 OrderPlacedEvent::NAME => 'onStoreOrder',
+                // you can also subscribe this way if you pass only
+                // the event object as first argument and omit the second
+                // of the $dispatcher->dispatch method
+                OrderPlacedEvent::class => 'onStoreOrder',
             ];
         }