[Security] Fix incorrect checkPostAuth() signature in UserChecker documentation

[lacatoire]

The documentation for UserChecker::checkPostAuth() currently shows the following signature:

public function checkPostAuth(UserInterface $user, TokenInterface $token): void

This is incorrect for Symfony 7.3, where the $token argument is:

• nullable
• optional

and handled internally via func_get_arg() for backward compatibility

The correct signature for 7.3 should be:

public function checkPostAuth(UserInterface $user, ?TokenInterface $token = null): void

[javiereguiluz]

It's tre. You can check this here: https://github.com/symfony/symfony/blob/7.3/src/Symfony/Component/Security/Core/User/UserCheckerInterface.php

Thanks Louis-Arnaud for fixing this issue and for perfectly explaining the issue.