Updating doc for runtime-extension flag

cmd/clusterctl/cmd/init.go

@@ -105,7 +105,7 @@ func init() {
 	initCmd.PersistentFlags().StringSliceVar(&initOpts.ipamProviders, "ipam", nil,
 		"IPAM providers and versions (e.g. infoblox:v0.0.1) to add to the management cluster.")
 	initCmd.PersistentFlags().StringSliceVar(&initOpts.runtimeExtensionProviders, "runtime-extension", nil,
-		"Runtime extension providers and versions (e.g. test:v0.0.1) to add to the management cluster.")
+		"Runtime extension providers and versions to add to the management cluster; please note that clusterctl doesn't include any default runtime extensions and thus it is required to use custom configuration files to register runtime extensions.")
 	initCmd.PersistentFlags().StringSliceVar(&initOpts.addonProviders, "addon", nil,
 		"Add-on providers and versions (e.g. helm:v0.1.0) to add to the management cluster.")
 	initCmd.Flags().StringVarP(&initOpts.targetNamespace, "target-namespace", "n", "",