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

TODO: do the same thing tonet.Socket if this one is good to go.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

net, doc