Mention the debug_exception_response_format config in guides

guides/source/api_app.md

@@ -163,6 +163,14 @@ class definition:
 config.api_only = true
 ```
 
+Optionally, in `config/environments/development.rb` add the following line
+to render error responses using the API format (JSON by default) when it
+is a local request:
+
+```ruby
+config.debug_exception_response_format = :api
+```
+
 Finally, inside `app/controllers/application_controller.rb`, instead of:
 
 ```ruby