Not sure if I am getting your question correctly.
Yes by default VLAN 1 is allowed on all uplinks of fabric as well as in the DL2 network and it cannot be segregated to any of the fabric uplink port or port-channel. You can use another Vlan as a native vlan but you need to define the same at the vnic level (South bound traffic). By default Vlan 1 will be set as native on all the uplinks.
The FI will never receive any untagged traffic either it will receive the traffic with tagged Vlan 1 by default or if we have defined any other native vlan over the vnic let say 10 then the traffic will be tagged as 10.
Can you briefly describe exactly what documentation you are looking for and if you can elaborate your question?