PCI/PM: Expand description of pci_set_power_state()

Add two reasons for returning 0 value to the description of
pci_set_power_state() to include the cases when:

  - the transition is to D1 or D2 but D1 and D2 are not supported
  - the transition is to D3 but D3 is not supported
Signed-off-by: Piotr Gregor <piotrgregor@rsyncme.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

drivers/pci/pci.c

@@ -892,7 +892,9 @@ EXPORT_SYMBOL_GPL(__pci_complete_power_transition);
  * -EINVAL if the requested state is invalid.
  * -EIO if device does not support PCI PM or its PM capabilities register has a
  * wrong version, or device doesn't support the requested state.
+ * 0 if the transition is to D1 or D2 but D1 and D2 are not supported.
  * 0 if device already is in the requested state.
+ * 0 if the transition is to D3 but D3 is not supported.
  * 0 if device's power state has been successfully changed.
  */
 int pci_set_power_state(struct pci_dev *dev, pci_power_t state)