PCI: Clarify policy for vendor IDs in pci.txt

Clarify pci.txt so it matches the "do not add new entries unless they are
shared between multiple drivers" comment in include/linux/pci_ids.h.

[bhelgaas: changelog, strengthen language]
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>