Mark mandatory elevator functions in the biodoc.txt

biodoc.txt mentions that elevator functions marked with * are mandatory, but
no function is marked with *. Mark the 3 functions which should be
implemented by any io scheduler.

Signed-off-by: Nikanth Karthikesan <knikanth@suse.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>