Update azure-cni-overlay.md

Add clarification about supporting adding subnet for expansion with CNI Overlay  @palma21

Author: srinman

articles/aks/azure-cni-overlay.md

@@ -43,7 +43,7 @@ Like Azure CNI Overlay, Kubenet assigns IP addresses to pods from an address spa
 
 ## IP address planning
 
-- **Cluster Nodes**: Cluster nodes go into a subnet in your VNet, so verify you have a subnet large enough to account for future scale. A simple `/24` subnet can host up to 251 nodes (the first three IP addresses in a subnet are reserved for management operations).
+- **Cluster Nodes**: Cluster nodes go into a subnet in your VNet, so verify you have a subnet large enough to account for future scale. Cluster can't scale to another subnet but you can add new nodepools in another subnet within the same VNet for expansion. A simple `/24` subnet can host up to 251 nodes (the first three IP addresses in a subnet are reserved for management operations).
 
 - **Pods**: The overlay solution assigns a `/24` address space for pods on every node from the private CIDR that you specify during cluster creation. The `/24` size is fixed and can't be increased or decreased. You can run up to 250 pods on a node. When planning the pod address space, ensure that the private CIDR is large enough to provide `/24` address spaces for new nodes to support future cluster expansion.