uapi: fix ioctl documentation

The description of the BLKGETNRZONES zoned block device ioctl was not
added as a comment together with this ioctl definition in commit
65e4e3ee ("block: Introduce BLKGETNRZONES ioctl"). Add its
description here.
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/uapi/linux/blkzoned.h

@@ -138,6 +138,7 @@ struct blk_zone_range {
  * @BLKRESETZONE: Reset the write pointer of the zones in the specified
  *                sector range. The sector range must be zone aligned.
  * @BLKGETZONESZ: Get the device zone size in number of 512 B sectors.
+ * @BLKGETNRZONES: Get the total number of zones of the device.
  */
 #define BLKREPORTZONE	_IOWR(0x12, 130, struct blk_zone_report)
 #define BLKRESETZONE	_IOW(0x12, 131, struct blk_zone_range)