Fix description of %util in iostat and sar manual pages

Description of %util in iostat manual page was not accurate, because it
is not percentage of CPU time, but percentage of elapsed time during
which I/O requesrt were issued to the device.
Also make sar manual page consistent with that of iostat.

Signed-off-by: Sebastien GODARD <sysstat@users.noreply.github.com>