[Validator] Documentation for group sequences

[jalliot]

Some doc must be added about @GroupSequence and @GroupSequenceProvider.
It seems that only @bschussek knows them well enough at the moment.

See symfony/symfony#3893 and symfony/symfony#2947.

[sstok]

@bschussek ping

[wouterj]

ping @bschussek , can you please provide some more information about this (or even create a PR for this)?

[wouterj]

closing this as #2647 is merged