doc: Clarify scope of eviction protection of outbound block-relay peers
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 [[https://github.com/bitcoin/bitcoin/pull/19871 | core#19871]]

Test Plan: proofreading

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

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

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