Updates UDN docs to include required NS label

Author: Steven Smith

modules/nw-udn-best-practices.adoc

@@ -31,3 +31,14 @@ Before setting up a `UserDefinedNetwork` (UDN) resource, users should consider t
 
 * The cluster subnet and services CIDR for a UDN cannot overlap with the default cluster subnet CIDR. OVN-Kubernetes network plugin uses `100.64.0.0/16` as the default network's join subnet, you must not use that value to configure a UDN `joinSubnets` field. If the default address values are used anywhere in the cluster's network you must override it by setting the `joinSubnets` field. For more information, see "Additional configuration details for a UserDefinedNetworks CR".
 
+* {product-title} administrators should be aware that empty `matchLabel` selectors results in selecting all namespaces of the cluster.
+
+* For primary networks, the namespace used for the `UserDefinedNetwork` CR must include the `k8s.ovn.org/primary-user-defined-network` label. This label cannot be updated, and can only be added when the namespace is created. The following conditions apply with the `k8s.ovn.org/primary-user-defined-network` namespace label:
+
+** If the namespace is missing the `k8s.ovn.org/primary-user-defined-network` label and a pod is created, the pod attaches itself to the default network.
+
+** If the namespace is missing the `k8s.ovn.org/primary-user-defined-network` label and a primary UDN CR is created that matches the namespace, the UDN reports an error status and the network will not be created.
+
+** If the namespace is missing the `k8s.ovn.org/primary-user-defined-network` label and a primary UDN already exists, a pod in the namespace will be created and attached to the default network.
+
+** If the namespace _has_ the label, and a primary UDN does not exist, a pod in the namespace will not be created until the UDN is created.

modules/nw-udn-cr.adoc

@@ -14,6 +14,10 @@ The following procedure creates a user-defined network that is namespace scoped.
 //If any cluster default networked pods exist before the user-defined network is created, any further pods created in this namespace will return an error message: `What_is_this`.
 //====
 
+.Prerequisites
+
+* You have created a namespace with the `k8s.ovn.org/primary-user-defined-network` label.
+
 .Procedure
 
 . Create a request for either a `Layer2` or `Layer3` topology type user-defined network: