Update NZBGet settings

 - Added a short description of the script (NZBGet shows this in the edit download dialog; the current paragraph is too long to show there).
 - Add a hint for NZBGet that the all_tracks option is True/False, so that it shows a switch.

Only a couple of tiny changes, as I have only just been through the setup of this script. Thanks for this though; much easier to setup than similar scripts that do a lot more (which aren't really needed anymore).