Finish documenting generic APIs

There are several parts missing from generic API docs: https://solitude.readthedocs.org/en/latest/topics/generic.html

I just spent longer than I care to admit trying to run `api.generic.products.get_object_or_404(public_id=...)` instead of `api.generic.product.get_object_or_404(public_id=...)` because things like that are hard to look up in the code itself.
