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 <[e-mail]>
Signed-off-by: Jerome Brunet <[e-mail]>
Signed-off-by: Manivannan Sadhasivam <[e-mail]>
Signed-off-by: Bjorn Helgaas <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

Documentation/PCI/endpoint/pci-vntb-howto.rst

···
 attributes that can be configured by the user::
 
 	# ls functions/pci_epf_vntb/func1/pci_epf_vntb.0/
-	db_count    mw1         mw2         mw3         mw4         num_mws
-	spad_count
+	ctrl_bar  db_count  mw1_bar  mw2_bar  mw3_bar  mw4_bar	spad_count
+	db_bar	  mw1	    mw2      mw3      mw4      num_mws	vbus_number
+	vntb_vid  vntb_pid
 
 A sample configuration for NTB function is given below::
 
···
 	# echo 128 > functions/pci_epf_vntb/func1/pci_epf_vntb.0/spad_count
 	# echo 1 > functions/pci_epf_vntb/func1/pci_epf_vntb.0/num_mws
 	# echo 0x100000 > functions/pci_epf_vntb/func1/pci_epf_vntb.0/mw1
+
+By default, each construct is assigned a BAR, as needed and in order.
+Should a specific BAR setup be required by the platform, BAR may be assigned
+to each construct using the related ``XYZ_bar`` entry.
 
 A sample configuration for virtual NTB driver for virtual PCI bus::