[doc] Update comments
Summary:
Maintain comments on the external interfaces rather than on the internal functions that implement them.
Partial backport of core#22950:
https://github.com/bitcoin/bitcoin/pull/22950/commits/29727c2aa1233f7c5b91a17884c405e0aef10c6e
Depends on D12334.
Test Plan: Read the comments
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Differential Revision: https://reviews.bitcoinabc.org/D12335