doc: fix unit of zone_reset_threshold and relation to other option

doc: fix unit of zone_reset_threshold and relation to other option

The zone_reset_threshold option uses the 'sectors with data' accounting
then it was described to have 'logical block' as its unit. However, the
accounting was implemented with 'byte' unit. Fix the description of the
option.

Also, the zone_reset_threshold option works together with the
zone_reset_frequency option. Describe this relation also.

Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Reviewed-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>