doc(netxlite): revamp the documentation (#523)

Part of https://github.com/ooni/probe-cli/pull/506. In parallel with
tutorials, we also need to make sure we have good documentation.

internal/netxlite/dnsx/dnsx.go

@@ -1,14 +1,14 @@
-// Package dnsx contains the dnsx model.
+// Package dnsx contains DNS extension types.
 package dnsx
 
-// HTTPSSvc is an HTTPSSvc reply.
+// HTTPSSvc is the reply to an HTTPS DNS query.
 type HTTPSSvc struct {
-	// ALPN contains the ALPNs inside the HTTPS reply
+	// ALPN contains the ALPNs inside the HTTPS reply.
 	ALPN []string
 
-	// IPv4 contains the IPv4 hints.
+	// IPv4 contains the IPv4 hints (which may be empty).
 	IPv4 []string
 
-	// IPv6 contains the IPv6 hints.
+	// IPv6 contains the IPv6 hints (which may be empty).
 	IPv6 []string
 }