[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.
Let's add the whole XML format:

````xml
<?xml version="1.0" ?>
<srv:container xmlns="http://symfony.com/schema/dic/security"
    xmlns:srv="http://symfony.com/schema/dic/services">

    <config>
        <firewall name="secured_area" pattern="^/">
            <x509 provider="your_user_provider"/>
        </firewall>
    </config>
</srv:container>
````