ax(node): replace prose comment with usage example on rebuildKDTree

AX principle 2: comments show HOW (usage example), not WHAT
(which the function name already says).

Co-Authored-By: Charon <charon@lethean.io>

pkg/node/peer.go

@@ -556,8 +556,7 @@ func (r *PeerRegistry) Count() int {
 	return len(r.peers)
 }
 
-// rebuildKDTree rebuilds the KD-tree from current peers.
-// Must be called with lock held.
+// r.rebuildKDTree() // called after AddPeer, UpdatePeer, RemovePeer, UpdateMetrics — lock must be held
 func (r *PeerRegistry) rebuildKDTree() {
 	if len(r.peers) == 0 {
 		r.kdTree = nil