[Routing] Add a note for refering invokable controllers

Author: chalasr

routing.rst

@@ -531,6 +531,11 @@ and method: ``AppBundle\Controller\BlogController::showAction``. But if you
 follow some simple conventions, the logical name is more concise and allows
 more flexibility.
 
+.. tip::
+
+   For referring to the ``__invoke`` method of a controller, no need for
+   passing the method name, just use its fully-qualified namespace.
+
 .. note::
 
    In addition to using the logical name or the fully-qualified class name,