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

Documentation/filesystems/proc.txt

@@ -1089,8 +1089,8 @@ The "processes" line gives the number  of processes and threads created, which
 includes (but  is not limited  to) those  created by  calls to the  fork() and
 clone() system calls.
 
-The  "procs_running" line gives the  number of processes  currently running on
-CPUs.
+The "procs_running" line gives the total number of threads that are
+running or ready to run (i.e., the total number of runnable threads).
 
 The   "procs_blocked" line gives  the  number of  processes currently blocked,
 waiting for I/O to complete.