PCI: Document reset method return values

The pci_reset_function() path may try several different reset methods:
device-specific resets, PCIe Function Level Resets, PCI Advanced Features
Function Level Reset, etc.

Add a comment about what the return values from these methods mean.  If one
of the methods fails, in some cases we want to continue and try the next
one in the list, but sometimes we want to stop trying.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>