[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