OCPBUGS-57625: Added warning note to br-ex doc for reserved SDN int names

Author: dfitzmau

modules/creating-manifest-file-customized-br-ex-bridge.adoc

@@ -48,6 +48,20 @@ If you require an environment with a single network interface controller (NIC) a
 After you install {op-system-first} and the system reboots, the Machine Config Operator injects Ignition configuration files into each node in your cluster, so that each node received the `br-ex` bridge network configuration. To prevent configuration conflicts, the `configure-ovs.sh` shell script receives a signal to not configure the `br-ex` bridge.
 endif::postinstall-bare-metal[]
 
+[WARNING]
+====
+The OpenShift SDN network plugin was removed in {product-title} 4.17 but certain reserved interface names, such as `br0`, were not removed. {product-title} uses these reserved names for internal use only. If you attempt to use one of these reserved names with a new bridge interface that interacts with a customized `br-ex` bridge, the `nmstate` API fails and outputs an error message that is similiar the following example:
+
+[source,terminal,subs="verbatim"]
+----
+[2025-06-17T14:12:25Z INFO  nmstate::query_apply::net_state] Retrying on: VerificationError: Absent/Down interface br0/ovs-interface still found as OvsInterface(OvsInterface { base: BaseInterface { name: "br0", profile_name: None, description: None, iface_type: OvsInterface, driver: None, state: Ignore
+----
+
+Notice that the output has the `state` parameter set to `Ignore`.
+
+NetworkManager does not manage these reserved name and the `nmstate` API cannot create or delete a bridge interface that uses one of these reserved names.   
+====
+
 .Prerequisites
 ifndef::postinstall-bare-metal[]
 * Optional: You have installed the link:https://nmstate.io/[`nmstate`] API so that you can validate the NMState configuration.