minor #9235 Added a note about controller arguments (javiereguiluz)

This PR was merged into the 3.4 branch.

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Added a note about controller arguments

This finishes #8085.

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9075771 Added a note about controller arguments

Author: javiereguiluz

security/form_login_setup.rst

@@ -157,6 +157,12 @@ Great! Next, add the logic to ``loginAction()`` that displays the login form::
         ));
     }
 
+.. note::
+
+    If you get an error that the ``$authUtils`` argument is missing, it's
+    probably because you need to activate this new feature in Symfony 3.4. See
+    this :ref:`controller service argument note <controller-service-arguments-tag>`.
+
 Don't let this controller confuse you. As you'll see in a moment, when the
 user submits the form, the security system automatically handles the form
 submission for you. If the user submits an invalid username or password,