HOWTO: add offset unit info for offset= option

HOWTO: add offset unit info for offset= option

Add "in bytes" and "A percentage can be specified by ...",
based on an email post titled "in what units is offset option?".

-- example of offset in percentage
 # ./fio --debug=parse --name=xxxxx --ioengine=sync --rw=rw --size=1m --unlink=1 --offset=-51 | grep "SET start_offset_percent"
 parse    12859 SET start_offset_percent 50

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