Documentation: update /proc/stat "intr" count summary

The sum at the beginning of line "intr" includes also unnumbered
interrupts.  It implies that the sum at the beginning isn't the sum
of the remainder of the line, not even an estimation.

Fixed the documentation to mention that.

This behaviour was added to /proc/stat in commit a2eddfa9 ("x86:
make /proc/stat account for all interrupts")
Signed-off-by: Jan Moskyto Matejka <mq@suse.cz>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/filesystems/proc.txt

@@ -1245,8 +1245,9 @@ second).  The meanings of the columns are as follows, from left to right:
 
 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
-interrupts serviced; each  subsequent column is the  total for that particular
-interrupt.
+interrupts serviced  including  unnumbered  architecture specific  interrupts;
+each  subsequent column is the  total for that particular numbered interrupt.
+Unnumbered interrupts are not shown, only summed into the total.
 
 The "ctxt" line gives the total number of context switches across all CPUs.