Document how to use the CSRF REQUEST_TOKEN in Symfony forms

[ausi]

https://github.com/contao/contao/pull/3214

$this->createFormBuilder(…, [
    'csrf_field_name' => 'REQUEST_TOKEN',
    'csrf_token_manager' => $manager, // @contao.csrf.token_manager
    'csrf_token_id' => $tokenName, // %contao.csrf_token_name%
]);