doc: update fio doc for xnvme engine

- Elaborate about the various sync, async and admin
  interfaces.
- add missing io_uring_cmd async backend entry.
- xnvme ioengine doesn't support file stat.

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
Link: https://lore.kernel.org/r/20220816053821.440-3-ankit.kumar@samsung.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
2 files changed