Doc: better explanation of procs_running

the description in Documentation/filesystems/proc.txt of the
procs_running entry in /proc/stat is confusing (according to that
description, it looks as if procs_running could only be a number
between 0 and the number of CPUs).

Changed it to a more accurate description in the patch attached.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>