PCI/PM: Add ABI document for sysfs file d3cold_allowed

This patch adds ABI document for the following sysfs file:

/sys/bus/pci/devices/.../d3cold_allowed

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>