Add missing documentation of 'int dir' parameters.

This integer parameter appears in all of the game's anim_length,
flash_length and redraw methods, but the documentation only described
it properly in the section for anim_length.

The section for flash_length refers you to anim_length for the
description of all the parameters, but unhelpfully, it did so without
a conveniently clickable (in HTML) cross-reference.

And the section for game_redraw missed out the description of 'int
dir' completely, which is particularly unhelpful since that's the
function most likely to actually need to care about it! (Even if
forward and reversed move animations look different, they _probably_
still have the same duration, so it's more likely that anim_length
would ignore 'dir' and redraw would use it than vice versa.)