[#73962736] Document public methods with Rdoc

Document the public API methods using Rdoc.

Note I have not documented `NetLaunch#new` as there was nothing I could
add that would not repeat the documentation for `NetLaunch#run` and I
think it's simple enough so as to be self-explanatory.

Also, YARD (though not Rdoc) will automatically document it as:

> A new instance of NetLaunch.

* * *

For discussion; please let me know if this is too simplistic.

This isn't quite right. `new` returns a new object. `initialize` is executed by Ruby on the new object. So this method doesn't return a `Cli` instance which handles parsing, this method actually sets some instance variables and then calls the `parse` method, i.e. this method initiates parsing of the command-line arguments.