minor #6951 [Routing] Add a note for invokable controllers (chalasr)

This PR was merged into the 2.7 branch.

Discussion
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[Routing] Add a note for invokable controllers

Since this feature can help to avoid defining controllers as services or writing `Controller::__invoke` when it's not needed, I think we should document it.

Commits
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0b046b3 [Routing] Add a note for refering invokable controllers

Author: xabbuh

routing.rst

@@ -535,6 +535,12 @@ and method: ``AppBundle\Controller\BlogController::showAction``. But if you
 follow some simple conventions, the logical name is more concise and allows
 more flexibility.
 
+.. tip::
+
+   To refer to an action that is implemented as the ``__invoke()`` method of a controller class,
+   you do not have to pass the method name, but can just use the fully qualified class name (e.g.
+   ``AppBundle\Controller\BlogController``).
+
 .. note::
 
    In addition to using the logical name or the fully-qualified class name,