CSRF warning docs on Request::enableHttpMethodParameterOverride()

[HttpFoundation] fixed the docs so that it gives some explanation about how you are vulnerable to CSRF when you enable the httpMethodeParameterOverride
[Session] remove invalid workaround in session regenerate

The original issue #7380 was just caused because the developer missed to save the session before doing the redirect. That's all. This reverts #8270 and following.
[HttpFoundation] Minor spelling fix in PHPDocs
bug #12326 [Session] remove invalid hack in session regenerate (Tobion)

This PR was merged into the 2.3 branch.

Discussion
----------

[Session] remove invalid hack in session regenerate

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The original issue #7380 was just caused because the developer missed to save the session before doing the redirect. That's all. Such mistakes won't happen anymore with #12341

This reverts #8270 and following. Also it makes absolutely no sense to do this only for the `files` save handler which creates huge inconsistencies. All save handlers are affected and it's more a documentation thing.

Commits
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703d906 [Session] remove invalid workaround in session regenerate
minor #12168 [HttpFoundation] CSRF warning docs on Request::enableHttpMethodParameterOverride() (Kristof Van Cauwenbergh)

This PR was merged into the 2.3 branch.

Discussion
----------

[HttpFoundation] CSRF warning docs on Request::enableHttpMethodParameterOverride()

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12043
| License       | MIT
| Doc PR        | /

Since I wanted to understand this issue I did some research and altered the comment block. Is this a clear enough explanation or does it need more?

Commits
-------

deb70ab CSRF warning docs on Request::enableHttpMethodParameterOverride()
Merge branch '2.3' into 2.5

* 2.3:
  added missing files
  [TwigBundle] added a test
  Indicate which file was being parsed if an exception is thrown while running translation:debug
  [ClassLoader] Cast $useIncludePath property to boolean
  [HttpFoundation] Minor spelling fix in PHPDocs
  improve error message for multiple documents
  [Session] remove invalid workaround in session regenerate
  [Kernel] ensure session is saved before sending response
  [Routing] serialize the compiled route to speed things up
  [Validator] Fixed Regex::getHtmlPattern() to work with complex and negated patterns
  [DependencyInjection] use inheritdoc for loaders
  [Config] fix filelocator with empty name
  [Form] fix form handling with unconventional request methods like OPTIONS
  CSRF warning docs on Request::enableHttpMethodParameterOverride()

Conflicts:
	src/Symfony/Component/Routing/Route.php