AVI file format documentation

[Andrew Talbot]

While we are on the subject of AVI files: could someone please take a look
at the function IAVIStream_fnWriteData() in avifil32/avifile.c? There is an
unused variable "dwPos" (line 1326), which has been there since this
function was first implemented (2002-10-18), I could just remove this
variable, but I think the calculation of available space may be incorrect
anyway. Comments please!

Thanks,

-- 
Andy.