PCI: Update pci_find_slot() description in pci.txt

pci_find_slot() has been removed from the kernel.  Document its
closest replacement, pci_get_domain_bus_and_slot(), instead.

[bhelgaas: fix indentation, keep mention in "obsolete" section]
Signed-off-by: Yijing Wang <wangyijing@huawei.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>