update prompt

Author: m-chau

src/azure-cli/azure/cli/command_modules/acs/custom.py

@@ -1286,8 +1286,10 @@ def aks_upgrade(cmd,
                 return None
             upgrade_all = True
         else:
-            msg = ("Since control-plane-only argument is specified, this will upgrade only the control plane to {}. "
-                   "Node pool will not change. Continue?").format(instance.kubernetes_version)
+            msg = ("Since control-plane-only argument is specified, this will upgrade only the kubernetes version of the control plane to {}. "
+                   "Kubernetes versions of the node pools will remain unchanged, "
+                   "but node image version may be upgraded if there has been cluster config change that requires VM reimage. "
+                   "Continue?").format(instance.kubernetes_version)
             if not yes and not prompt_y_n(msg, default="n"):
                 return None