[net processing] Improve documentation for Peer destruction/locking

Suggested here:

- https://github.com/bitcoin/bitcoin/pull/19607#discussion_r467071878
- https://github.com/bitcoin/bitcoin/pull/19829#discussion_r546116786

src/net_processing.cpp

@@ -791,6 +791,11 @@ void PeerManager::FinalizeNode(const CNode& node, bool& fUpdateConnectionTime) {
     LOCK(cs_main);
     int misbehavior{0};
     {
+        // We remove the PeerRef from g_peer_map here, but we don't always
+        // destruct the Peer. Sometimes another thread is still holding a
+        // PeerRef, so the refcount is >= 1. Be careful not to do any
+        // processing here that assumes Peer won't be changed before it's
+        // destructed.
         PeerRef peer = RemovePeer(nodeid);
         assert(peer != nullptr);
         misbehavior = WITH_LOCK(peer->m_misbehavior_mutex, return peer->m_misbehavior_score);

src/net_processing.h

@@ -46,6 +46,8 @@ struct CNodeStateStats {
  * Memory is owned by shared pointers and this object is destructed when
  * the refcount drops to zero.
  *
+ * Mutexes inside this struct must not be held when locking m_peer_mutex.
+ *
  * TODO: move most members from CNodeState to this structure.
  * TODO: move remaining application-layer data members from CNode to this structure.
  */
@@ -210,7 +212,8 @@ private:
       * on extra block-relay-only peers. */
     bool m_initial_sync_finished{false};
 
-    /** Protects m_peer_map */
+    /** Protects m_peer_map. This mutex must not be locked while holding a lock
+     *  on any of the mutexes inside a Peer object. */
     mutable Mutex m_peer_mutex;
     /**
      * Map of all Peer objects, keyed by peer id. This map is protected