proc: document `guest' column in /proc/stat

We added a new column in cpuX lines of /proc/stat, to show the amount of
time spent by a cpu servicing a guest, without updating
Documentation/filesystems/proc.txt

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -1032,9 +1032,9 @@ Various pieces   of  information about  kernel activity  are  available in the
 since the system first booted.  For a quick look, simply cat the file:
 
   > cat /proc/stat
-  cpu  2255 34 2290 22625563 6290 127 456 0
-  cpu0 1132 34 1441 11311718 3675 127 438 0
-  cpu1 1123 0 849 11313845 2614 0 18 0
+  cpu  2255 34 2290 22625563 6290 127 456 0 0
+  cpu0 1132 34 1441 11311718 3675 127 438 0 0
+  cpu1 1123 0 849 11313845 2614 0 18 0 0
   intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
   ctxt 1990473
   btime 1062191376
@@ -1056,6 +1056,7 @@ second).  The meanings of the columns are as follows, from left to right:
 - irq: servicing interrupts
 - softirq: servicing softirqs
 - steal: involuntary wait
+- guest: running a guest
 
 The "intr" line gives counts of interrupts  serviced since boot time, for each
 of the  possible system interrupts.   The first  column  is the  total of  all