[Serializer] Doc for groups support

| Q                | A
| --------------- | ---
| Doc fix?      | no
| New docs? | yes symfony/symfony#12092
| Applies to   | 2.7

This is the documentation for serialization groups support.
I would remove these 3 variants. I think it's clear enough (and if it isn't, we could put something in comments). The current way takes up too much space for quite a simple task.