control/controlclient: fix priority of DERP server, add comment

control/controlclient/direct.go

@@ -544,7 +544,7 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 			c.logf("debug: adding DERP endpoints to all peers")
 			for i := range nm.Peers {
 				peer := &nm.Peers[i]
-				peer.Endpoints = append(peer.Endpoints, "127.3.3.40:1")
+				peer.Endpoints = append([]string{"127.3.3.40:1"}, peer.Endpoints...)
 			}
 		}
 		for _, profile := range resp.UserProfiles {

wgengine/magicsock/magicsock.go

@@ -758,7 +758,7 @@ func (c *Conn) LinkChange() {
 // AddrSet is a set of UDP addresses that implements wireguard/device.Endpoint.
 type AddrSet struct {
 	publicKey key.Public    // peer public key used for DERP communication
-	addrs     []net.UDPAddr // ordered priority list provided by wgengine
+	addrs     []net.UDPAddr // ordered priority list (low to high) provided by wgengine
 
 	mu sync.Mutex // guards roamAddr and curAddr