added template attribute (#34917)

* added template reference

* added more clarification

* quick format nit fix

extensions/amp-render/amp-render.md

@@ -131,6 +131,10 @@ Your endpoint must implement the requirements specified in the [CORS Requests in
 
 The `src` attribute may be omitted if the `[src]` attribute exists. `[src]` supports URL and non-URL expression values.
 
+### `template`
+
+References an ID of a defined templating element. This attribute is not necessary if the template is a child of the `amp-render` element.
+
 ### `credentials`
 
 Defines a `credentials` option as specified by the [Fetch API](https://fetch.spec.whatwg.org/).