update --reschedule documentation to optional computation of cpu schedule

update --reschedule documentation to optional computation of cpu schedule

Before 3bbfbda (cpu: optionally use computed schedule,
Tue Mar 20 15:35:03 2012 +0100), the cpu backend did not perform
any scheduling and the --reschedule option was not taken into account.
The commit changed this behavior but did not update the documentation
of the --reschedule option.  Update the documentation.

Signed-off-by: Sven Verdoolaege <skimo@kotnet.org>