OCPBUGS-43683: Clarify MTU configuration in SR-IOV network node policy

Author: amolnar-rh

modules/nw-sriov-networknodepolicy-object.adoc

@@ -57,11 +57,14 @@ To avoid a node in an unhealthy MCP from blocking the application of node networ
 
 <5> Optional: The priority is an integer value between `0` and `99`. A smaller value receives higher priority. For example, a priority of `10` is a higher priority than `99`. The default value is `99`.
 
-<6> Optional: The maximum transmission unit (MTU) of the virtual function. The maximum MTU value can vary for different network interface controller (NIC) models.
+<6> Optional: The maximum transmission unit (MTU) of the physical function and all its virtual functions. The maximum MTU value can vary for different network interface controller (NIC) models.
 +
 [IMPORTANT]
 ====
 If you want to create virtual function on the default network interface, ensure that the MTU is set to a value that matches the cluster MTU.
+
+If you want to modify the MTU of a single virtual function while the function is assigned to a pod, leave the MTU value blank in the SR-IOV network node policy.
+Otherwise, the SR-IOV Network Operator reverts the MTU of the virtual function to the MTU value defined in the SR-IOV network node policy, which might trigger a node drain.
 ====
 
 <7> Optional: Set `needVhostNet` to `true` to mount the `/dev/vhost-net` device in the pod. Use the mounted `/dev/vhost-net` device with Data Plane Development Kit (DPDK) to forward traffic to the kernel network stack.