[Form] Add doc for FormEvents
compare version using PHP_VERSION_ID

To let opcode caches optimize cached code, the `PHP_VERSION_ID`
constant is used to detect the current PHP version instead of calling
`version_compare()` with `PHP_VERSION`.
Azerbaijani locale
minor #12496 [Form] Add doc for FormEvents (GromNaN)

This PR was merged into the 2.3 branch.

Discussion
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[Form] Add doc for FormEvents

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

Provide the same level of phpdoc for form events than we have for other event classes.

http://api.symfony.com/2.3/Symfony/Component/HttpKernel/KernelEvents.html
http://api.symfony.com/2.3/Symfony/Component/Form/FormEvents.html

Commits
-------

656d45f [Form] Add doc for FormEvents
Merge branch '2.3' into 2.5

* 2.3:
  [SecurityBundle] Authentication entry point is only registered with firewall exception listener, not with authentication listeners
  be smarter when guessing the document root
  Azerbaijani locale
  Fixed grammar error in docblock
  Adjust upgrade file rendering
  [Bridge/Propel1] Changed deps to accepts all upcoming propel1 versions
  compare version using PHP_VERSION_ID
  [Form] Add doc for FormEvents
  don't override internal PHP constants

Conflicts:
	UPGRADE-3.0.md
	src/Symfony/Bundle/FrameworkBundle/Command/ServerRunCommand.php
	src/Symfony/Component/Debug/ErrorHandler.php
	src/Symfony/Component/HttpFoundation/Response.php