[PATCH] Documentation: CPU load calculation description

Describes how/when the information exported to `/proc/stat' is calculated,
and possible problems with this approach.

Signed-off-by: Vassili Karpov <av1474@comtv.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>