blk-cgroup: Fix doc related to blkcg_exit_queue

Since 4cf6324b, a portion of function blk_cleanup_queue was moved to
a newly created function called blk_exit_queue, including the call of
blkcg_exit_queue. So, adjust the documenation according.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>