aio: Comment use of IOCB_FLAG_IOPRIO aio flag

Comment the use of the IOCB_FLAG_IOPRIO aio flag similarly to the
IOCB_FLAG_RESFD flag.
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Reviewed-by: NJohannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: NDamien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/uapi/linux/aio_abi.h

@@ -50,6 +50,8 @@ enum {
  *
  * IOCB_FLAG_RESFD - Set if the "aio_resfd" member of the "struct iocb"
  *                   is valid.
+ * IOCB_FLAG_IOPRIO - Set if the "aio_reqprio" member of the "struct iocb"
+ *                    is valid.
  */
 #define IOCB_FLAG_RESFD		(1 << 0)
 #define IOCB_FLAG_IOPRIO	(1 << 1)