[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    | (or 2.6+)
| Fixed tickets | no

Some documentation for pre authenticated firewalls, and added remote_user configuration reference for this new firewall.
missing backtick here (spotted by the Travis build)