feature #4319 Documentation for debug:event-dispatcher command (matthieuauger)

This PR was merged into the master branch.

Discussion
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Documentation for debug:event-dispatcher command

See #4230

As my english is far from perfect, I tried to make it short. Any feedback/suggestions welcome

Commits
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a4fed11 Documentation for debug:event-dispatcher command

Author: weaverryan

Diff for: cookbook/service_container/event_listener.rst

@@ -133,3 +133,20 @@ done as follow::
     Two types of request are available in the :class:`Symfony\\Component\\HttpKernel\\HttpKernelInterface`
     interface: ``HttpKernelInterface::MASTER_REQUEST`` and
     ``HttpKernelInterface::SUB_REQUEST``.
+
+Debugging Event Listeners
+-------------------------
+
+You can find out what listeners are registered in the event dispatcher
+using the console. To show all events and their listeners, run:
+
+.. code-block:: bash
+
+    $ php app/console debug:event-dispatcher
+
+You can get registered listeners for a particular event by specifying
+its name:
+
+.. code-block:: bash
+
+    $ php app/console debug:event-dispatcher kernel.exception