[Serializer] Name Converter

| Q                | A
| --------------- | ---
| Doc fix?      | no
| New docs? | yes symfony/symfony#13120
| Applies to   | 2.7

This is the documentation for the Serializer Name Converter feature.
I think "first name" is must more common (at least, for people that are not natives). I didn't even know about "given name" before. It's also what we use in the rest of the docs