Adds limitation to UDN docs

Author: Steven Smith

modules/nw-udn-limitations.adoc

@@ -26,4 +26,6 @@ While user-defined networks (UDN) offer highly customizable network configuratio
 
 * *Default network service access*: A user-defined network pod is isolated from the default network, which means that most default network services are inaccessible. For example, a user-defined network pod cannot currently access the {product-title} image registry. Because of this limitation, source-to-image builds do not work in a user-defined network namespace. Additionally, other functions do not work, including functions to create applications based on the source code in a Git repository, such as `oc new-app <command>`, and functions to create applications from an {product-title} template that use source-to-image builds. This limitation might also affect other `openshift-*.svc` services.
 
-* *Connectivity limitation*: NodePort services on user-defined networks are not guaranteed isolation. For example, NodePort traffic from a pod to a service on the same node is not accessible, whereas traffic from a pod on a different node succeeds.
+* *Connectivity limitation*: NodePort services on user-defined networks are not guaranteed isolation. For example, NodePort traffic from a pod to a service on the same node is not accessible, whereas traffic from a pod on a different node succeeds.
+
+* *Unclear error message for IP address exhaustion*: When the subnet of a user-defined network runs out of available IP addresses, new pods fail to start. When this occurs, the following error is returned: `Warning: Failed to create pod sandbox`. This error message does not clearly specify that IP depletion is the cause. To confirm the issue, you can check the *Events* page in the pod's namespace on the {product-title} web console, where an explicit message about subnet exhaustion is reported.