document classes and overrides used in the example app

[n2ygk]

Usage documentation and the example app are not consistent with each other. There are several classes used in the example that lack proper usage documentation and docstrings.

Documentation updates (or changes to use documented classes and deprecation of undocumented classes) are needed for the following:

• relations.SerializerMethodResourceRelatedField
• relations.SerializerMethodHyperlinkedRelatedField
• get_queryset() and get_object() implementation differs between example views and usage doc.

There may be others; please update this list if you see them.