added a doc link

Author: fabpot

app/config/config.yml

@@ -26,6 +26,7 @@ framework:
     trusted_hosts:   ~
     trusted_proxies: ~
     session:
+        # http://symfony.com/doc/current/reference/configuration/framework.html#handler-id
         handler_id:  session.handler.native_file
         save_path:   "%kernel.root_dir%/../var/sessions/%kernel.environment%"
     fragments:       ~