Adding info about login throttling

Author: ThomasLandauer

security/custom_authenticator.rst

@@ -168,6 +168,11 @@ can define what happens in these cases:
     useful for e.g. login forms, where the login controller is run again
     with the login errors.
 
+    If you're using :ref:`login throttling <security-login-throttling>`,
+    you can check if ``$exception`` is an instance of
+    :class:`Symfony\\Component\\Security\\Core\\Exception\\TooManyLoginAttemptsAuthenticationException`
+    (e.g. to display an appropriate message).
+
     **Caution**: Never use ``$exception->getMessage()`` for ``AuthenticationException``
     instances. This message might contain sensitive information that you
     don't want to be publicly exposed. Instead, use ``$exception->getMessageKey()``