Add gRPC Python API Documentation Sanity Checks

It is currently the responsibility of developers to remember to add each new object added to the surface API to [`grpc.rst`](https://github.com/grpc/grpc/blob/dd4830eae80143f5b0a9a3a1a024af4cf60e7d02/doc/python/sphinx/grpc.rst). It is entirely possible for this to be forgotten, especially as the project changes hands.

One solution to this problem would be to automatically generated the docs. However, as @lidizheng pointed out to me, we cannot do this *and* maintain logical human-curated categories within the docs.

An alternative would be to add a simple sanity check ensuring that every element of the surface API is represented in the docs.