Clarify test_order default configuration behaviour [ci skip]

Explain config.active_support.test_order defaults
to :random in newly-generated applications thanks
to generated config/environments/test.rb, otherwise
default is :sorted (until Rails 5.0).

guides/source/configuring.md

@@ -524,7 +524,7 @@ There are a few configuration options available in Active Support:
 
 * `config.active_support.bare` enables or disables the loading of `active_support/all` when booting Rails. Defaults to `nil`, which means `active_support/all` is loaded.
 
-* `config.active_support.test_order` sets the order that test cases are executed. Possible values are `:sorted` and `:random`. Currently defaults to `:sorted`. In Rails 5.0, the default will be changed to `:random` instead.
+* `config.active_support.test_order` sets the order that test cases are executed. Possible values are `:random` and `:sorted`. This option is set to `:random` in `config/environments/test.rb` in newly-generated applications. If you have an application that does not specify a `test_order`, it will default to `:sorted`, *until* Rails 5.0, when the default will become `:random`.
 
 * `config.active_support.escape_html_entities_in_json` enables or disables the escaping of HTML entities in JSON serialization. Defaults to `false`.