[net processing] Add comment about destructing Peer objects

Suggested here: bitcoin#19607 (comment)

src/net_processing.cpp

@@ -912,6 +912,10 @@ void PeerLogicValidation::FinalizeNode(NodeId nodeid, bool& fUpdateConnectionTim
     LOCK(cs_main);
     int misbehavior{0};
     {
+        // We remove the Peer object from g_peer_map here, but it's not guaranteed that
+        // we'll destruct Peer here since another thread could potentially still hold
+        // a reference to the object. Be careful not to do any processing here that
+        // assumes Peer won't be changed before it's destructed.
         PeerRef peer = GetPeerRef(nodeid);
         assert(peer != nullptr);
         misbehavior = WITH_LOCK(peer->m_misbehavior_mutex, return peer->m_misbehavior_score);