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

AX principle 2: comments show HOW with real values, not WHAT the
signature already says.

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

pkg/node/controller.go

@@ -21,7 +21,8 @@ type Controller struct {
 	pending map[string]chan *Message // message ID -> response channel
 }
 
-// NewController creates a new Controller instance.
+// ctrl := node.NewController(nodeManager, peerRegistry, transport)
+// rtt, err := ctrl.PingPeer("abc123def456")
 func NewController(node *NodeManager, peers *PeerRegistry, transport *Transport) *Controller {
 	c := &Controller{
 		node:      node,