misc: pci_endpoint_test: Document policy about adding pci_device_id

misc: pci_endpoint_test: Document policy about adding pci_device_id

Add a comment suggesting that if the endpoint controller Vendor and Device
ID are programmable, an existing entry might be usable for testing without
having to add an entry to pci_endpoint_test_tbl[].

Link: https://lore.kernel.org/linux-pci/20240611125057.1232873-6-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Frank Li <Frank.Li@nxp.com>