Document security.switch_user event

... in the cookbook article about How to Impersonate a User.

Added code sample about how to change the locale in case of a sticky locale:
http://symfony.com/doc/current/cookbook/session/locale_sticky_session.html

cookbook/security/impersonating_user.rst

@@ -151,3 +151,49 @@ setting:
                 ),
             ),
         ));
+        
+Events
+------
+
+The firewall dispatches the ``security.switch_user`` event right after the impersonation
+completed. The ``SwitchUserEvent`` is passed to the listener, based on which you are able
+to get the target user you impersonate.
+
+The cookbook article about
+:doc:`Making the Locale "Sticky" during a User's Session </cookbook/session/locale_sticky_session>`
+does not take any changing locale into account. The following code sample will show how to change the sticky locale:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # app/config/services.yml
+        services:
+            app.switch_user_listener:
+                class: AppBundle\EventListener\SwitchUserListener
+                tags:
+                    - { name: kernel.event_listener, event: security.switch_user, method: onSwitchUser }
+
+.. caution::
+
+    The listener implementation assumes your ``User`` entity has ``getLocale()``.
+
+.. code-block:: php
+
+        // src/AppBundle/EventListener/SwitchUserListener.pnp
+        
+        use Symfony\Component\Security\Http\Event\SwitchUserEvent;
+        
+        class SwitchUserListener
+        {
+            /**
+             * @param SwitchUserEvent $event
+             */
+            public function onSwitchUser(SwitchUserEvent $event)
+            {
+                $event->getRequest()->getSession()->set(
+                    '_locale', 
+                    $event->getTargetUser()->getLocale()
+                );
+            }
+        }