[Process] Update use of Process constants
Improving Web server configuration
kernel.bundle_dirs doesn't exist in KernelParameters
Improved Luhn validator doc

Based on feedback from @WouterJ on the Iban validator doc.
See https://github.com/symfony/symfony-docs/pull/2521
Improved CardScheme validator doc

Based on feedback from @WouterJ on the Iban validator doc.
See https://github.com/symfony/symfony-docs/pull/2521
Fix code blocks in contributing/code/git.rst
Fix duplicated form widget
Changing translations examples to not use single character variable name
fix typo in auto_start
Fix custom Roles in entity_provider cookbook.

The documentation seems to assume the implementation present in commit
https://github.com/symfony/symfony/pull/1673, which reverted soon after due
to a potential, but undisclosed security hole (citation @schmittjoh in https://github.com/symfony/symfony/commit/af70ac8d777873c49347ac828a817a400006cbea).

This incorrect documentation has likely been the source of many
of the following issues:
* symfony/symfony#1538 - [ACL RoleSecurityIdentity] check if instance of Role
* symfony/symfony#1748 - Replace Role to RoleInterface for RoleSecurityIdentity
* symfony/symfony#4309 - Issue related to custom group (role) and ACL/ACE
* symfony/symfony#5026 - potential bug in Symfony\Component\Security\Acl\Domain\RoleSecurityIdentity
* symfony/symfony#5076 - [Acl] altered the behaviour of RoleSecurityIdentity
* symfony/symfony#5171 - Fix/role security identity
* symfony/symfony#5303 - [Security] Check for RoleInterface instead of Role object in RoleSecurityIdentity
* symfony/symfony#5909 - Allow Custom Roles to implement the RoleInterface
* symfony/symfony#6012 - Securityidentity fix
Added a note to the dom-crawler docs about character sets
Added missing UniqueEntity ignoreNull documentation
Moved version directive before heading
Merge pull request #2508 from maxromanovsky/improving_web_server_config

Improving Web server configuration
Merge remote-tracking branch 'origin/2.0' into 2.0
[#2508] Adding extra warning about app_dev.php and config.php
Grammer tweak

Old grammar doesn't make sense.
Merge pull request #2511 from BenoitLeveque/2.0

kernel.bundle_dirs doesn't exist in KernelParameters
Merge pull request #2505 from romainneutron/process-constants

[Process] Update use of Process constants
Update acl.rst

minor fix for coding standards