block: fix uapi blkzoned.h comments

Update the kdoc comments for struct blk_zone (capacity field description
missing) and for struct blk_zone_report (flags field description
missing).

Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>