add document for temporary API:DNSResolverFunc, DNSResolverFunc

transport/internet/system_dns_android.go

@@ -3,14 +3,19 @@
 package internet
 
 import (
-	"context"
 	"net"
 )
 
 const SystemDNS = "8.8.8.8:53"
 
+/* DNSResolverFunc
+   This is a temporary API and is subject to removal at any time.
+*/
 type DNSResolverFunc func() *net.Resolver
 
+/* NewDNSResolver
+   This is a temporary API and is subject to removal at any time.
+*/
 var NewDNSResolver DNSResolverFunc = func() *net.Resolver {
 	return &net.Resolver{
 		PreferGo: true,