Clarify documentation for runtime parameter

Clarify documentation for runtime parameter

I realize this is highly subjective but I think the description of the
runtime parameter could be made a bit more precise. I misinterpreted its
meaning after reading the doc and only learned of my mistake by trial and
error using fio.  Either I'm just slow or the description could use
just a little more precision :)

Signed-off-by: Adam Horshack (horshack@live.com)