doc: various improvements to net.md

* Improve general description of the module, specifically,
  explain that it provides TCP or local communications
  (domain sockets on UNIX, named pipes on Windows) functionalities.
* Improve explanation of `allowHalfOpen`
* Nest the overloaded `server.listen()` API in a list, explain
  the common arguments and notes in the same place.

Some minor improvements:

* Add description to the `net.Server` constructor
* Add type annotations to `server.listen()`
* Add contexts to method links

PR-URL: #11636
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>