block: document BLK_STS_AGAIN usage

Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=2083917



commit 98d40e76
Author: Hannes Reinecke <hare@suse.de>
Date:   Tue May 24 07:56:30 2022 +0200

    block: document BLK_STS_AGAIN usage

    BLK_STS_AGAIN should only be used if RQF_NOWAIT is set and the bio
    would block. So we'd better document that to avoid accidental misuse.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
    Link: https://lore.kernel.org/r/20220524055631.85480-2-hare@suse.de


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

Signed-off-by: Ming Lei <ming.lei@redhat.com>