blk-wbt: Fix some comments

wbt_wait() no longer uses a spinlock as a parameter. Update the function
comments accordingly.

RWB_UNKNOWN_BUMP is used when we gradually adjust scale_steps toward the
center state, which is a value of 0.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250213100611.209997-2-yizhou.tang@shopee.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>