I have to keep documenting my own game...

comments are the way to go, i tend to comment everything,
my globals for exemple are commented, about why they are there and what they are supposed to keep tracks off

also i'm alway making some kind of documentations on functions in my comments just before declaration of the function... like that
so this way i can alway know the use of the function even some month later , i can also re-use the function in another project as the syntax is documented without guessing... what i done some month ago

; <name>(<arg1><arg2><arg3>) <arg1> is that
;                            <arg2> is that
;                            <arg3> is that
; the functon return <var> that contain <for what use>

function <name>(<arg1><arg2><arg3>)
...
...
end function