PCI: Expand the "PF" acronym in Kconfig help text

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>

drivers/pci/Kconfig

@@ -73,9 +73,9 @@ config PCI_PF_STUB
 	depends on PCI_IOV
 	help
 	  Say Y or M here if you want to enable support for devices that
-	  require SR-IOV support, while at the same time the PF itself is
-	  not providing any actual services on the host itself such as
-	  storage or networking.
+	  require SR-IOV support, while at the same time the PF (Physical
+	  Function) itself is not providing any actual services on the
+	  host itself such as storage or networking.
 
 	  When in doubt, say N.