null_blk: describe the usage of fault injection param

As null_blk is a very good start point to test block layer, this patch
adds description and comments to 'timeout', 'requeue' and 'init_hctx' to
explain how to use fault injection with null_blk.

The nvme has similar with nvme_core.fail_request in the form of comment.

Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>