[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 just do ``:method:`Symfony\\Component\\Serializer\\Normalizer\\GetSetMethodNormalizer::setIgnoredAttributes` ``