Adds important note to UDN docs

Author: Steven Smith

modules/nw-udn-best-practices.adoc

@@ -19,7 +19,8 @@ Before setting up a `UserDefinedNetwork` (UDN) resource, users should consider t
 +
 [IMPORTANT]
 ====
-For {product-title} 4.17 and later, clusters use `169.254.0.0/17` for IPv4 and `fd69::/112` for IPv6 as the default masquerade subnet. These ranges should be avoided by users. For updated clusters, there is no change to the default masquerade subnet.
+* For {product-title} 4.17 and later, clusters use `169.254.0.0/17` for IPv4 and `fd69::/112` for IPv6 as the default masquerade subnet. These ranges should be avoided by users. For updated clusters, there is no change to the default masquerade subnet.
+* Changing the cluster's masquerade subnet is unsupported after a user-defined network has been configured for a project. Attempting to modify the masquerade subnet after a UDN has been set up can disrupt the network connectivity and cause configuration issues.
 ====
 // May be something that is downstream only.
 //* No active primary UDN managed pod can also be a candidate for `v1.multus-cni.io/default-network`