This patch adds ABI document for the following sysfs file:

/sys/bus/pci/devices/.../d3cold_allowed
Signed-off-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Acked-by: NRafael J. Wysocki <rjw@sisk.pl>