Mention default values for `debug_exception_response_format` in configuring guide [ci skip]

guides/source/configuring.md

@@ -98,7 +98,7 @@ application. Accepts a valid week day symbol (e.g. `:monday`).
 
 * `config.exceptions_app` sets the exceptions application invoked by the ShowException middleware when an exception happens. Defaults to `ActionDispatch::PublicExceptions.new(Rails.public_path)`.
 
-* `config.debug_exception_response_format` sets the format used in responses when errors occur in development mode.
+* `config.debug_exception_response_format` sets the format used in responses when errors occur in development mode. Defaults to `:api` for API only apps and `:default` for normal apps.
 
 * `config.file_watcher` is the class used to detect file updates in the file system when `config.reload_classes_only_on_change` is true. Rails ships with `ActiveSupport::FileUpdateChecker`, the default, and `ActiveSupport::EventedFileUpdateChecker` (this one depends on the [listen](https://github.com/guard/listen) gem). Custom classes must conform to the `ActiveSupport::FileUpdateChecker` API.