[WIP][WCM][Security] Added remote_user firewall info and documentation for pre authenticated firewalls

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#10698)
| Applies to    | 2.6+
| Fixed tickets | no

Some documentation for pre authenticated firewalls, and added remote_user configuration reference for this new firewall.
I prefer to just use "User provider" here instead of linking to an interface. "You will need to either use an available user provider" reads much better than "You will need to either use an available UserProviderInterface"