blkcg: Fix kernel-doc warnings

Avoid that the following warnings are reported when building with W=1:

block/blk-cgroup.c:1755: warning: Function parameter or member 'q' not described in 'blkcg_schedule_throttle'
block/blk-cgroup.c:1755: warning: Function parameter or member 'use_memdelay' not described in 'blkcg_schedule_throttle'
block/blk-cgroup.c:1779: warning: Function parameter or member 'blkg' not described in 'blkcg_add_delay'
block/blk-cgroup.c:1779: warning: Function parameter or member 'now' not described in 'blkcg_add_delay'
block/blk-cgroup.c:1779: warning: Function parameter or member 'delta' not described in 'blkcg_add_delay'

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>