doc: update about size

doc: update about size

In few cases with fio option size the number of bytes of data transferred
is actually less than what we specified. This can happen if there are
gaps or holes while doing I/O's or if we are running a mix of sequential
and random workload.
Update the documentation for that.

Fixes: https://github.com/axboe/fio/issues/1486

Signed-off-by: Ankit Kumar <ankit.kumar@samsung.com>
Signed-off-by: Vincent Fu <vincent.fu@samsung.com>