doc: refer to BIPs 339/155 in feature negotiation

and add fSuccessfullyConnected doxygen documentation
to clarify that it is set to true on VERACK

src/net.h

@@ -445,6 +445,7 @@ public:
      * messages, implying a preference to receive ADDRv2 instead of ADDR ones.
      */
     std::atomic_bool m_wants_addrv2{false};
+    /** fSuccessfullyConnected is set to true on receiving VERACK from the peer. */
     std::atomic_bool fSuccessfullyConnected{false};
     // Setting fDisconnect to true will cause the node to be disconnected the
     // next time DisconnectNodes() runs

src/net_processing.cpp

@@ -2745,12 +2745,11 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
         return;
     }
 
-    // Feature negotiation of wtxidrelay must happen between VERSION and VERACK
-    // to avoid relay problems from switching after a connection is up.
+    // BIP339 defines feature negotiation of wtxidrelay, which must happen between
+    // VERSION and VERACK to avoid relay problems from switching after a connection is up.
     if (msg_type == NetMsgType::WTXIDRELAY) {
         if (pfrom.fSuccessfullyConnected) {
-            // Disconnect peers that send wtxidrelay message after VERACK; this
-            // must be negotiated between VERSION and VERACK.
+            // Disconnect peers that send a wtxidrelay message after VERACK.
             LogPrint(BCLog::NET, "wtxidrelay received after verack from peer=%d; disconnecting\n", pfrom.GetId());
             pfrom.fDisconnect = true;
             return;
@@ -2769,10 +2768,11 @@ void PeerManagerImpl::ProcessMessage(CNode& pfrom, const std::string& msg_type,
         return;
     }
 
+    // BIP155 defines feature negotiation of addrv2 and sendaddrv2, which must happen
+    // between VERSION and VERACK.
     if (msg_type == NetMsgType::SENDADDRV2) {
         if (pfrom.fSuccessfullyConnected) {
-            // Disconnect peers that send SENDADDRV2 message after VERACK; this
-            // must be negotiated between VERSION and VERACK.
+            // Disconnect peers that send a SENDADDRV2 message after VERACK.
             LogPrint(BCLog::NET, "sendaddrv2 received after verack from peer=%d; disconnecting\n", pfrom.GetId());
             pfrom.fDisconnect = true;
             return;