[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