[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
add description for the `validation_groups` option
swapped comment and opening in xml configuration example
[#4003] A few more form_themes config changes
Make a small grammatical adjustment

The original form of the phrase - «The other option is to specifically
checking if a session has expired after the session is started.» - is
not entirely grammatically correct, as the infinitive form of «to check»
shall normally be used, or the gerund form, which is the form used in
this commit.
[Cookbook][External Parameters] Enhance description of environment variables
change misleading language identifier
changed dash to underscore
formatting to follow 72 characters line limit
Update security.rst
minor #4285 Update security.rst (placid2000)

This PR was submitted for the 2.5 branch but it was merged into the 2.4 branch instead (closes #4285).

Discussion
----------

Update security.rst

Hotfix: allow_if option might been added afterwards and seems to miss in the description above the list

Commits
-------

6a0551c Update security.rst
support Varnish in configuration blocks
bug #4284 change misleading language identifier (Kristof Van Cauwenbergh, kristofvc)

This PR was merged into the 2.3 branch.

Discussion
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change misleading language identifier

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets | #4251

Commits
-------

db85c67 formatting to follow 72 characters line limit
12cccc8 changed dash to underscore
d68cd09 change misleading language identifier
feature #4246 [Reference] add description for the `validation_groups` option (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Reference] add description for the `validation_groups` option

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes (symfony/symfony#6528)
| Applies to    | all
| Fixed tickets | #3358

Commits
-------

c41b17c add description for the `validation_groups` option
Merge branch '2.3' into 2.5

* 2.3:
  formatting to follow 72 characters line limit
  changed dash to underscore
  change misleading language identifier
  add description for the `validation_groups` option
Merge branch '2.5'

* 2.5:
  formatting to follow 72 characters line limit
  changed dash to underscore
  change misleading language identifier
  add description for the `validation_groups` option