Form login docs missing info about success_handler and failure_handler

[michnovka]

https://symfony.com/doc/current/security/form_login.html

We should add info that this is configurable at least to this page. At the moment the only mention of this is under the login link docs - https://symfony.com/doc/current/security/login_link.html#using-the-login-link-authenticator

This is where the main security docs page leads under its Customize Successful and Failed Authentication Behavior section.

I was working on this feature in my project and the only way I found out that this is usable even for login form was from the source code.