[Security] Added remote_user firewall info and documentation for pre authenticated firewalls
[Reference][Configuration] Removed version added for remote_user
rebased using x509 pr, added remote_user pre authenticated part
fix missing backtick, rephrased bottom note
fixes missing backtick
swapped comment and opening in xml configuration example
uppercase title
feature #3912 [Security] Added remote_user firewall info and documentation for pre authenticated firewalls (Maxime Douailin, mdouailin)

This PR was merged into the master branch.

Discussion
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[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.

Commits
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f36c45e uppercase title
e6aa733 swapped comment and opening in xml configuration example
b8a0eb2 fixes missing backtick
be0d866 fix missing backtick, rephrased bottom note
86ba188 rebased using x509 pr, added remote_user pre authenticated part
8465d46 [Reference][Configuration] Removed version added for remote_user
34ad1b5 [Security] Added remote_user firewall info and documentation for pre authenticated firewalls