Mention api custom template need for respond_with

Fixes spree/spree#4967

content/api/summary.md

@@ -107,3 +107,9 @@ parameter along with the request:
 `http://example.com/api/products/1?template=small_show`. The API component of
 Spree will then detect this parameter, find the template, and then use this to
 render the response.
+
+***
+Due to [the way this implemented](https://github.com/spree/spree/blob/v2.3.1/api/lib/spree/api/responders/rabl_template.rb#L5-L18)
+you need to ensure the action rendering in your custom template explicitly
+calls `respond_with`
+***