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

Block-relay-only peers were introduced by bitcoin#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 deleted comment
leans to think of.

src/net_processing.cpp

@@ -2021,8 +2021,6 @@ static void ProcessHeadersMessage(CNode& pfrom, CConnman& connman, ChainstateMan
         if (!pfrom.fDisconnect && pfrom.IsFullOutboundConn() && nodestate->pindexBestKnownBlock != nullptr) {
             // If this is an outbound full-relay peer, check to see if we should protect
             // it from the bad/lagging chain logic.
-            // Note that block-relay-only peers are already implicitly protected, so we
-            // only consider setting m_protect for the full-relay peers.
             if (g_outbound_peers_with_protect_from_disconnect < MAX_OUTBOUND_PEERS_TO_PROTECT_FROM_DISCONNECT && nodestate->pindexBestKnownBlock->nChainWork >= ::ChainActive().Tip()->nChainWork && !nodestate->m_chain_sync.m_protect) {
                 LogPrint(BCLog::NET, "Protecting outbound peer=%d from eviction\n", pfrom.GetId());
                 nodestate->m_chain_sync.m_protect = true;