[PATCH] io/storage: Documentation update to as-iosched.txt

Documentation update, adding references to CFQ scheduler and to another
document about selecting IO Schedulers.

Signed-off-by: Filipe Lautert <filipe@icewall.org>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>