Unittest for non-native validation only fields + use case added to serializer docs

I tested starting from 2.3.13 moving backward  all the way to 2.3.5 to see where just overriding the restore_object() to handle the non-native fields was sufficient enough without breaking the browsable API. None of the versions worked so I have come to believe that forms in browsable API were not ever (within the aforementioned versions) including the non-native fields, and to_native() must be overridden to achieve this.

With this pull request:
1. I have added a use case to the serializer documentation to help people use this feature
2. I have added a unittest so moving forward we don't unintentionally break this process
3. I have tested (Travis CI) the unit test and pep8 verified the file (--ignore=E128,E225,E501)

Will we natively support validation-only fields in the near future?  Well, if so, then we would just update the unittest as well as the documentation to reflect the changes.