zonefs: update documentation to reflect zone size vs capacity

Update the zonefs documentation to reflect the difference between a zone's
size and it's capacity.

The maximum file size in zonefs is the zones capacity, for ZBC and ZAC
based devices, which do not have a separate zone capacity, the zone
capacity is equal to the zone size.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>