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

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:

c8feb44c3ad8081984ec71f96241fdbdb0b68ef0

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>