Documentation: PCI: endpoint: Document BAR assignment

It is now possible to assign BARs while creating a vNTB endpoint function.
Update the documentation accordingly.

Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Signed-off-by: Manivannan Sadhasivam <mani@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Link: https://patch.msgid.link/20250731-vntb-doc-v1-1-95a0e0cd28d0@baylibre.com