[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.

Author: Antoine Riard

src/net_processing.cpp

@@ -321,10 +321,17 @@ struct CNodeState {
      */
     bool fSupportsDesiredCmpctVersion;
 
-    /** State used to enforce CHAIN_SYNC_TIMEOUT
-      * Only in effect for outbound, non-manual, full-relay connections, with
-      * m_protect == false
-      * Algorithm: if a peer's best known block has less work than our tip,
+    /** State used to enforce CHAIN_SYNC_TIMEOUT and EXTRA_PEER_CHECK_INTERVAL logic.
+      *
+      * Both are only in effect for outbound, non-manual, non-protected connections.
+      * Any peer protected (m_protect = true) is not chosen for eviction. A peer is
+      * marked as protected if all of these are true:
+      *   - its connection type is IsBlockOnlyConn() == false
+      *   - it gave us a valid connecting header
+      *   - we haven't reached MAX_OUTBOUND_PEERS_TO_PROTECT_FROM_DISCONNECT yet
+      *   - it has a better chain than we have
+      *
+      * CHAIN_SYNC_TIMEOUT: if a peer's best known block has less work than our tip,
       * set a timeout CHAIN_SYNC_TIMEOUT seconds in the future:
       *   - If at timeout their best known block now has more work than our tip
       *     when the timeout was set, then either reset the timeout or clear it
@@ -334,6 +341,9 @@ struct CNodeState {
       *     and set a shorter timeout, HEADERS_RESPONSE_TIME seconds in future.
       *     If their best known block is still behind when that new timeout is
       *     reached, disconnect.
+      *
+      * EXTRA_PEER_CHECK_INTERVAL: after each interval, if we have too many outbound peers,
+      * drop the outbound one that least recently announced us a new block.
       */
     struct ChainSyncTimeoutState {
         //! A timeout used for checking whether our peer has sufficiently synced