PM/PCI: Update PCI power management documentation

I power management document, Documentation/power/pci.txt, is
outdated and partially inaccurate.  It also is missing some important
information about the power management of PCI device.  Rewrite it to
make it more up to date and more complete.
Reviewed-by: NRandy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: NRafael J. Wysocki <rjw@sisk.pl>