Documentation: document ublk user recovery feature

Add documentation for user recovery feature of ublk subsystem.

Signed-off-by: ZiyangZhang <ZiyangZhang@linux.alibaba.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20221018045346.99706-2-ZiyangZhang@linux.alibaba.com


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