explain in Readme why can't upgrade var.kubernetes_version and var.orchestrator_version at the same time

Author: zioproto

examples/multiple_node_pools/README.md

@@ -72,3 +72,42 @@ kubectl get nodes # check nodes version
 
 In the `az aks list` output you will have `KubernetesVersion` and `CurrentKubernetesVersion` both at 1.27.7
 The control plane version will be 1.27.7 and the nodes will be 1.27.7.
+
+## Note on Issue #465
+
+The current implementation does not allow to upgrade `var.kubernetes_version` and `var.orchestrator_version` at the same time.
+
+We can test at this point a simultaneous upgrade to 1.28.3:
+
+```
+terraform apply -var="kubernetes_version=1.28.3" -var="orchestrator_version=1.28.3"
+```
+This will generate a plan where the azure_kubernetes_cluster resource is updated in place and the system node pool is updated.
+
+```
+  # module.aks.azurerm_kubernetes_cluster.main will be updated in-place
+  ~ resource "azurerm_kubernetes_cluster" "main" {
+        id                                  = "/subscriptions/<redacted>/resourceGroups/4c273d71bc7898d6-rg/providers/Microsoft.ContainerService/managedClusters/prefix-4c273d71bc7898d6-aks"
+        name                                = "prefix-4c273d71bc7898d6-aks"
+        tags                                = {}
+        # (29 unchanged attributes hidden)
+
+      ~ default_node_pool {
+            name                         = "nodepool"
+          ~ orchestrator_version         = "1.27.7" -> "1.28.3"
+            tags                         = {}
+            # (22 unchanged attributes hidden)
+        }
+
+        # (4 unchanged blocks hidden)
+    }
+```
+
+that will fail with the following error:
+
+```
+│ Error: updating Default Node Pool Agent Pool (Subscription: "<redacted>"
+│ Resource Group Name: "4c273d71bc7898d6-rg"
+│ Managed Cluster Name: "prefix-4c273d71bc7898d6-aks"
+│ Agent Pool Name: "nodepool") performing CreateOrUpdate: agentpools.AgentPoolsClient#CreateOrUpdate: Failure sending request: StatusCode=0 -- Original Error: Code="NodePoolMcVersionIncompatible" Message="Node pool version 1.28.3 and control plane version 1.27.7 are incompatible. Minor version of node pool version 28 is bigger than control plane version 27. For more information, please check https://aka.ms/aks/UpgradeVersionRules"
+```