ublk_drv: comment on ublk_driver entry of Kconfig

Add help info for choosing to build ublk_drv as module or builtin.
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20221029010432.598367-3-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/Kconfig

@@ -408,6 +408,12 @@ config BLK_DEV_UBLK
 	  definition isn't finalized yet, and might change according to future
 	  requirement, so mark is as experimental now.
 
+	  Say Y if you want to get better performance because task_work_add()
+	  can be used in IO path for replacing io_uring cmd, which will become
+	  shared between IO tasks and ubq daemon, meantime task_work_add() can
+	  can handle batch more effectively, but task_work_add() isn't exported
+	  for module, so ublk has to be built to kernel.
+
 source "drivers/block/rnbd/Kconfig"
 
 endif # BLK_DEV