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