PCI/PM: Add kerneldoc description of pci_pm_reset()

The pci_pm_reset() function is not a very nice interface due to its
limitations and conditional behavior (e.g. it doesn't affect devices
in low-power states), but it cannot be simply dropped, because
existing device drivers may depend on it.  However, its behavior and
limitations should be well documented, so add an appropriate
kerneldoc comment to it.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>