Add docs for FlashMessageImportanceMapper

Author: benr77

notifier.rst

@@ -662,6 +662,30 @@ and
 :class:`Symfony\\Component\\Notifier\\Notification\\EmailNotificationInterface`
 also exists to modify messages sent to those channels.
 
+Customize Browser Notifications (Flash Messages)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The default behaviour for Browser channel notifications is to add a flash
+message with a key of "notification".
+
+However, it may be desirable to map the importance level of the notification
+to the type of flash message, so that you can style the different flash
+messages according to their importance.
+
+This can be done by overriding the default
+'notifier.flash_message_importance_mapper' service with your own implementation of
+:class:`Symfony\\Component\\Notifier\\FlashMessage\\FlashMessageImportanceMapperInterface`
+where you can provide your own "importance" to "alert level" mapping.
+
+Symfony currently provides an implementation for the Bootstrap CSS framework's
+typical alert levels, which you can implement immediately using:
+
+.. code-block:: yaml
+
+    # config/services.yaml
+    services:
+        notifier.flash_message_importance_mapper: Symfony\Component\Notifier\FlashMessage\BootstrapFlashMessageImportanceMapper
+
 Disabling Delivery
 ------------------