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

Summary:
This is a partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15759 | 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

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