docs: fully document the network control interface

For developers that wish to integrate retroarch into other software - a popular tactic for building custom firmware usable on portable gaming devices - having a well-documented network control interface will speed up development.

This change fully documents each command, adding a short description for the commands, any arguments and responses, as well as an example for how to trigger it. Additionally, it fixes a few typos (SHOW_MESG and NEPLAY_GAME_WATCH) as well as documents missing commands that were added since this document was first written.

Author: josegonzalez