[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.
the standard is to use "The suppor tof serialization and deserialization groups was introduced in Symfony 2.7." Can you please change it to follow this standard?