How the game's core mechanics work, what all the stats actually do, a list of status effects and what they do, all the current abilities in the game and what they do... stuff that could be used to put encounters together without just guesswork.
It's a shame JS doesn't have something like Doxygen - as you write the code, you put information into specially formatted comments, and then Doxygen extracts that information, the information about function calls and parameters from the actual source, and creates a document with call trees, function documentation, class documentation, etc.
I use it heavily in my professional life, and it makes creating the kind of documentation you want much better (assuming programmers are disciplined enough to actually supply the comments - but that's what DARTs are for.)
(DART - Developer Attitude Readjustment Tool, a tool in the same family as the LART, but heavier since developers really should know better).
I am curious - when you say "Coding is taking time" - is it really software architecture type work, or just "take this block of text make a bunch of write() calls out of it"?