[PATCH] PCI: update Documentation/power/pci.txt

[PATCH] PCI: update Documentation/power/pci.txt

That document was wrong on some things, misleading on others; this
fixes some of the issues I noticed.

However it probably needs to say that drivers for devices that implement
the PCI PM spec "should" always use pci_set_power_state() to reduce the
power usage.  If I get ambitions I might submit a patch to the PCI core
to print a nag message for drivers that don't do that.

Updates the PCI PM docs, better matching the specs and code.

  - List both D3 states (D3hot, D3cold) up front.

  - Clarify that suspend() methods should disable I/0 (including DMA)
    and IRQs; it's not optional.

  - More accurately describe resume(); there are common cases where
    device re-initialization isn't appropriate.  The previous text said
    re-init was always required; that's false.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>