rpc: Fail to return undocumented return values

Summary:
Currently a few return values are undocumented. This is causing confusion at the least. See for example core#18476

Fix this by treating it as an internal bug to return undocumented return values.

This is a backport of core#20459
Depends on D12716

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12717