doc: Clarify scope of eviction protection of outbound block-relay peers

Summary:

[doc] Clarify scope of eviction protection of outbound block-relay peers

Block-relay-only peers were introduced by #15759. According to its
author, it was intented to make them only immune to outbound peer
rotation-based eviction and not from all eviction as modified comment
leans to think of.

Clearly indicate that outbound block-relay peers aren't protected
from eviction by the bad/lagging chain logic.

[doc] Clarify semantic of peer's m_protect w.r.t to outbound eviction logics

The field m_protect is used to protect from eviction both by bad/lagging
chain and extra outbound peers logics. Outbound block-relay peers are
always excluded from this protection.

This is a backport of core#19871

Test Plan: proofreading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D10420