[WIP] Added documentation for GroupSequence

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | #1248

The GroupSequence feature has never been documented.

@bschussek Can you review this ? You made an explanation on this topic about this feature: http://stackoverflow.com/questions/9996618/symfony2-dependency-between-validation-asserts