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>