wgengine/magicsock: clarify pre-disco 'tailscale ping' error message

This change clarifies the error message when a user pings a peer that is using an outdated version of Tailscale.

wgengine/magicsock/magicsock.go

@@ -648,8 +648,8 @@ func (c *Conn) Ping(ip netaddr.IP, cb func(*ipnstate.PingResult)) {
 	}
 
 	dk, ok := c.discoOfNode[peer.Key]
-	if !ok {
-		res.Err = "no discovery key for peer (pre 0.100?)"
+	if !ok { // peer is using outdated Tailscale version (pre-0.100)
+		res.Err = "no discovery key for peer (pre Tailscale 0.100 version?). Try: ping 100.x.y.z"
 		cb(res)
 		return
 	}