Update the security documentation for Symfony 2.8

[jonasarts]

As 'intention' is no longer supported as security configuration since Symfony 2.8, this setting should now be replaced in the online documentation with the csrf_token_id setting (and not only in the upgrade file).

symfony/symfony#16722
http://symfony.com/doc/current/reference/configuration/security.html

Regards, Jonas

[wouterj]

This is fixed some weeks ago with #6152 Thanks for submitting this issue!