It varies greatly Sam. Because you are working mostly solo I recommend keeping it fairly simple, don't overcomplicate things for yourself so manage just one or two documents at the most.
Take a look at this link:
http://www.gamasutra.com/view/feature/33...ument_.php it contains four pages for you to read. Don't worry about reading the sections on page 3 and 4 about Cost Projection, Market, Technical and Legal Analysis for now. Just have a look at the parts on writing a Game Concept document to give you some idea about how to structure the documentation for yourself.
Here's a couple of small suggestions for you...
The game concept document is a summary of your game, an introduction that briefly but excitedly introduces the game plot, setting and so on. Another section that contains a list of the key features you plan to have, and perhaps a background section to list where you got your inspiration from for making the game, other games that influenced it and so on.
Now I appreciate that isn't a design document, its a concept, but a concept document allows you to remember what the key features are, what the genre is and its a foundation for your game.
Then write a design document (
http://accad.osu.edu/~pgerstma/class/gmz...cument.pdf ) that "details" the key features of your game (which are in the concept document) break them down, prioritise them, some features won't be as important as others. Explain "what" and "how" the player will interact with your game world, with the GUI and so on.
The design document is your blueprint to work from. You can include some pseudo-code in the design document but I don't recommend it, just detail the "mechanics" of your game, how it works basically
I hope that helps and I haven't bogged you down with more information than you need.