Documentation: nvme: add an example for nvme fault injection

This adds an example of how to inject errors into admin commands.

Suggested-by: Thomas Tai <thomas.tai@oracle.com>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Minwoo Im <minwoo.im@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>