documentation: zonefs: Document sysfs attributes

Document the max_wro_seq_files, nr_wro_seq_files, max_active_seq_files
and nr_active_seq_files sysfs attributes.

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>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>