qemu-options.hx: mention retrace= VGA option

The feature was added in commit cb5a7aa8c32141bb Sep 2008.
My description is based on "Better VGA retrace emulation (needed
for some DOS games/demos)" from
http://www.boblycat.org/~malc/code/patches/qemu/index.html

Signed-off-by: Matthew Ogilvie <mmogilvi_qemu@miniinfo.net>
Signed-off-by: malc <av1474@comtv.ru>
