[ci skip] Clarify load_paths behaviour

Clarify `I18n.load_paths` vs `Rails.application.config.i18n.load_paths`
behaviour.

[Gannon McGibbon + Alberto Almagro Sotelo]

guides/source/i18n.md

@@ -135,6 +135,8 @@ I18n.available_locales = [:en, :pt]
 I18n.default_locale = :pt
 ```
 
+Note that appending directly to `I18n.load_paths` instead of to the application's configured i18n will _not_ override translations from external gems.
+
 ### Managing the Locale across Requests
 
 The default locale is used for all translations unless `I18n.locale` is explicitly set.