docs: lack of documentation on some response types

[cmwaters]

From an external team:

Docs often don’t document what responses are. For example, the only reason I can think of to use GetProof is to verify the proof - but it’s documented as a completely opaque structure. We had to reverse engineer this including figuring out all the details about serialization, and a quick link to the spec would save a ton of work. Similarly, the docs give zero information about the expected contents of a shares response