mm: document /sys/devices/system/node/nodeX

Add a bare description of what /sys/devices/system/node/nodeX is.  Others
will follow in time but right now, none of that tree is documented.  The
existence of this file might at least encourage people to document new
entries.
Signed-off-by: NMel Gorman <mel@csn.ul.ie>
Reviewed-by: NKOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/ABI/stable/sysfs-devices-node

+What:		/sys/devices/system/node/nodeX
+Date:		October 2002
+Contact:	Linux Memory Management list <linux-mm@kvack.org>
+Description:
+		When CONFIG_NUMA is enabled, this is a directory containing
+		information on node X such as what CPUs are local to the
+		node.