PCI/PM: Add comments for PME poll support for PCIe

There are comments on why PME poll support is necessary for PCI
devices, but not for PCIe devices.  That may lead to misunderstanding
that PME poll is only necessary for PCI devices.  So add comments
related to PCIe PME poll to make it more clear.

The content of comments comes from the changelog of commit:

379021d5

Signed-off-by: Huang Ying <ying.huang@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>