docs: mark Async io_engine as dev preview
- Added `io_engine` to the pre-boot block device configuration.
Possible values: `Sync` (the default option) or `Async` (only available for
kernels newer than 5.10.51). The `Async` variant introduces a block device
- engine that uses io_uring for executing requests asynchronously.
+ engine that uses io_uring for executing requests asynchronously, which is in
+ **developer preview** (NOT for production use).
See `docs/api_requests/block-io-engine.md`.
- Added `block.io_engine_throttled_events` metric for measuring the number of
virtio events throttled because of the IO engine.
