Added documentation for allow_empty

Author: dantleech

bundles/routing_auto/token_providers.rst

@@ -9,6 +9,15 @@ Such values can be derived form the object for which the route is being
 generated or from the environment (e.g. you could use the current locale in
 the route).
 
+Global options
+--------------
+
+The following options apply to all token providers:
+
+``allow_empty``
+  Allow the token value to be empty. If the token value is empty then any
+  trailing slash will be removed, if the token is the last token, then the
+  leading slash will be removed. Default is ``false``
 
 ``content_method``
 ------------------