As far as I know, this is still the best place to find info, even though it is incomplete and may be out of date with the last couple changes.
There is the XML schema file that is part of the game if you know how to read one of those to find out what is valid...still that is only for the XML syntax, doesn't list any game variables.
There used to be an in-game command (/gvlist) to list all variables, but it has been removed for some reason
