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/doc.go

@@ -3,6 +3,9 @@
 // This package is the basic networking building block that you
 // should be using every time you need networking.
 //
+// You should consider checking the tutorial explaining how to use this package
+// for network measurements: https://github.com/ooni/probe-cli/tree/master/internal/tutorial/netxlite.
+//
 // Naming and history
 //
 // Previous versions of this package were called netx. Compared to such
@@ -27,13 +30,16 @@
 // We also want to map errors to OONI failures, which are described by
 // https://github.com/ooni/spec/blob/master/data-formats/df-007-errors.md.
 //
+// We want to have reasonable watchdog timeouts for each operation.
+//
 // Operations
 //
 // This package implements the following operations:
 //
 // 1. establishing a TCP connection;
 //
-// 2. performing a domain name resolution;
+// 2. performing a domain name resolution with the "system" resolver
+// (i.e., getaddrinfo on Unix) or custom DNS transports (e.g., DoT, DoH);
 //
 // 3. performing the TLS handshake;
 //