documentation: zonefs: Cleanup the mount options section

Use subsections to separate the descriptions of the "error=" and
"explicit-open" mount sections.

Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>