Documentation pass - disk IO

I've begun documenting the disk IO routines in Lusca-HEAD. I've started with the legacy disk routines in libiapp/disk.c - these are used by some of the early code (errorpages, icon loading, etc) and the UFS based swap log (ufs, aufs, diskd).

This code is relatively straight forward but I'm also documenting the API shortcomings so others (including myself!) will realise why it isn't quite good enough in its current form for processing fully asynchronous disk IO.

I'll make a pass through the async IO code on the flight back to Australia (so libasyncio/aiops.c and libasyncio/async_io.c) to document how the existing async IO code works and its shortcomings.