Tell users what a PCI PF is in the PCI_PF_STUB config help text.

Fixes: a8ccf8a6 ("PCI/IOV: Add pci-pf-stub driver for PFs that only enable VFs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Alexander Duyck <alexander.h.duyck@linux.intel.com>