[FrameworkBundle] Update the enabled_locales description

Author: javiereguiluz

reference/configuration/framework.rst

@@ -428,9 +428,11 @@ performance a bit:
             $framework->enabledLocales(['en', 'es']);
         };
 
-If some user makes requests with a locale not included in this option, the
-application won't display any error because Symfony will display contents using
-the fallback locale.
+An added bonus of defining the enabled locales is that they are automatically
+added as a requirement of the :ref:`special _locale parameter <routing-locale-parameter>`.
+For example, if you define this value as ``['ar', 'he', 'ja', 'zh']``, the
+``_locale`` routing parameter will have an ``ar|he|ja|zh`` requirement. If some
+user makes requests with a locale not included in this option, they'll see an error.
 
 set_content_language_from_locale
 ................................