PCI: Document pci_bus_for_each_resource()

There might be confusion about why pci_bus_for_each_resource() uses
Logical OR. Document the entire macro and explain how it works and why the
conditional needs to be like that.

Link: https://lore.kernel.org/r/20230330162434.35055-5-andriy.shevchenko@linux.intel.com


Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>