The user may wish to provide a dynamic queryset on a `RelatedField`
based on the `context`. The way to do that is to create a subclass of
`RelatedField` (or a child) and override the `get_queryset` method.
However, this is undocumented, and instantiating that field without a
`queryset` argument (because it's not needed) will raise an assertion
error.

Document `.get_queryset(self)` as an official part of the API of
`RelatedField`, and don't enforce the use of `queryset` when
`get_queryset` is overridden.