blk-wbt: doc: Update the doc of the wbt_lat_usec interface

The symbol wb_window_usec cannot be found. Update the doc to reflect the
latest implementation, in other words, the debugfs interface
'curr_win_nsec'.

Signed-off-by: Tang Yizhou <yizhou.tang@shopee.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20250727173959.160835-4-yizhou.tang@shopee.com


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