[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