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

include/linux/blk_types.h

@@ -105,6 +105,10 @@ typedef u16 blk_short_t;
 /* hack for device mapper, don't use elsewhere: */
 #define BLK_STS_DM_REQUEUE    ((__force blk_status_t)11)
 
+/*
+ * BLK_STS_AGAIN should only be returned if RQF_NOWAIT is set
+ * and the bio would block (cf bio_wouldblock_error())
+ */
 #define BLK_STS_AGAIN		((__force blk_status_t)12)
 
 /*