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 +48,13 @@ 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 some reserved interface names, such as `br0`, were not removed because {product-title} uses the names for internal use only. If you attempt to use one of these reserved names on a new bridge interface that interacts with a customized `br-ex` bridge, the `nmstate` API fails and outputs an error message that shows the `state` parameter for the bridge name set to `Ignore`.
+
+The NetworkManager service does not manage these reserved names 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.