Documentation: ABI: /sys/devices/system/cpu/cpu#/node

Describe NUMA node symlink created for CPUs when CONFIG_NUMA is set.

Cc: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>