minor #10467 Fix and improve JSON login docs (dunglas)

This PR was squashed before being merged into the 3.4 branch (closes #10467).

Discussion
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Fix and improve JSON login docs

Since symfony/symfony#22494, it's mandatory to return a valid response in the controller to prevent a 500 error.

I've updated the docs accordingly, and added an example of what can be returned.

This PR is similar to #9611 (that was right, sorry I just noticed).

ping @vincentchalamon

Commits
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570dce3 Fix and improve JSON login docs

Author: javiereguiluz

security/json_login_setup.rst

@@ -80,6 +80,12 @@ path:
              */
             public function loginAction(Request $request)
             {
+                $user = $this->getUser();
+
+                return $this->json(array(
+                    'username' => $user->getUsername(),
+                    'roles' => $user->getRoles(),
+                ));
             }
         }
 
@@ -117,11 +123,11 @@ path:
 
         return $routes;
 
-Don't let this empty controller confuse you. When you submit a ``POST`` request
-to the ``/login`` URL with the following JSON document as the body, the security
-system intercepts the requests. It takes care of authenticating the user with
-the submitted username and password or triggers an error in case the authentication
-process fails:
+When you submit a ``POST`` request to the ``/login`` URL with the following JSON document as the body,
+the security system intercepts the requests.
+It takes care of authenticating the user with the submitted username and password or triggers an error
+in case the authentication process fails.
+If the authentication is successful, the controller defined earlier will be executed. 
 
 .. code-block:: json