Add info how to update AKS cluster

Author: Edvin Norling

docs/xks/operator-guide/kubernetes/aks.md

@@ -90,6 +90,7 @@ export RG=rg1
 export POOL_NAME=default
 export CLUSTER_NAME=cluster1
 export AZURE_LOCATION=westeurope
+export KUBE_VERSION=1.21.9
 ```
 
 What AKS versions can I pick in this Azure location:
@@ -102,6 +103,16 @@ az aks get-versions --location $AZURE_LOCATION -o table
 az aks get-upgrades --resource-group $RG --name $CLUSTER_NAME --output table
 ```
 
+We recommend to only upgrade control-plane separately and then upgrade the nodes.
+
+```shell
+az aks upgrade --resource-group $RG --name $CLUSTER_NAME --kubernetes-version $KUBE_VERSION --control-plane-only
+```
+
+```shell
+az aks nodepool upgrade --resource-group $RG --cluster-name $CLUSTER_NAME --name $POOL_NAME --kubernetes-version $KUBE_VERSION
+```
+
 ### Upgrading node pools without upgrading cluster
 
 From time to time you might want to upgrade your Node Pools without upgrading the Kubernetes version. We always recommend to look at