tailcfg: update PeerAPIDNS Port value documentation

We do not intend to use this value for feature support communication in
the future, and have applied changes elsewhere that now fix the expected
value.

Updates tailscale/corp#19391
Updates tailscale/corp#20398

Signed-off-by: James Tucker <james@tailscale.com>

tailcfg/tailcfg.go

@@ -682,14 +682,12 @@ type Service struct {
 	//        node's Tailscale IPv6 address.
 	//     * "peerapi-dns-proxy": the local peerapi service supports
 	//        being a DNS proxy (when the node is an exit
-	//        node). For this service, the Port number is really
-	//        the version number of the service.
+	//        node). For this service, the Port number must only be 1.
 	Proto ServiceProto
 
 	// Port is the port number.
 	//
-	// For Proto "peerapi-dns", it's the version number of the DNS proxy,
-	// currently 1.
+	// For Proto "peerapi-dns", it must be 1.
 	Port uint16
 
 	// Description is the textual description of the service,