Document the new customization options for the security

[fabpot]

see symfony/symfony#6069

[fabpot]

If you think that my blog post about it (http://symfony.com/blog/new-in-symfony-2-4-customize-the-security-features-with-ease) would be a good addition to the cookbook, that's also an option.

[wouterj]

fixed by #2914 and #3357