My suggestion would be to start at the high level, and work your way down putting emphasis on things that aren't going to change or change much--that way, when you change things the docs won't become as wildly out of date as quickly.
So, document things like "What are the big conceptual pieces of my project?", "How does data flow around?", "If a user sees <X>, what's behind it?".
Then, if you feel it needful, dig down into function-level and deeper stuff.