doc: improve comments relating to block-relay-only peers

Summary:
This is a partial backport of Core PR15759 : https://github.com/bitcoin/bitcoin/pull/15759/commits/937eba91e1550bc3038dc541c236ac83e0a0e6d5

Depends on D6426

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D6434