psi: clarify the units used in pressure files

psi: clarify the units used in pressure files

The output of the PSI files show a bunch of numbers with no unit.  The
psi.txt documentation file also does not indicate what units are used.
One can only find out by looking at the source code.  The units are
percentage for the averages and useconds for the total.  Make the
information easier to find by documenting the units in psi.txt.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Waiman Long <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: Tejun Heo <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>