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

AX Principle 2: comments must show HOW with real values, not restate
what the type signature already says.

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

pkg/node/message.go

@@ -40,7 +40,8 @@ func IsProtocolVersionSupported(version string) bool {
 	return false
 }
 
-// MessageType defines the type of P2P message.
+// msg, err := NewMessage(MsgPing, identity.ID, peer.ID, PingPayload{SentAt: time.Now().UnixMilli()})
+// if msg.Type == MsgHandshake { handleHandshake(msg) }
 type MessageType string
 
 const (