Add details of file number/size related options to HOWTO

File number/size related options for each job/thread are difficult
to understand without looking at source.

fio(1) hasn't been updated (yet). There's already quite significant
amount of diff between these two, and people are basically only
updating HOWTO.

Signed-off-by: Tomohiro Kusumi <tkusumi@tuxera.com>
Signed-off-by: Jens Axboe <axboe@fb.com>