Document the effect of setting locale on a route with locale prefixes

When using localized routes and importing a collection of routes, the imported
routes can be prefixed with a different prefix for each locale.
This makes it easy to have localized routes for /en/about-us and /nl/over-ons
without having to define multiple routes.

This behavior is implemented in the
`Symfony\Component\Routing\Loader\Configurator\Traits\PrefixTrait` by replacing
the original route with a cloned version for each locale prefix.

However, any route which has a default value for the `_locale` parameter will
only be prefixed with the prefix for that locale and will not be cloned for
the other locale prefixes.

Author: nicwortel

routing.rst

@@ -2416,6 +2416,14 @@ with a locale. This can be done by defining a different prefix for each locale
             ;
         };
 
+.. tip::
+
+    If the special :ref:`_locale <routing-locale-parameter>` routing parameter
+    is set on any of the imported routes, that route will only be available
+    with the prefix for that locale. This is useful when you want to import
+    a collection of routes which contains a route that should only exist
+    in one of the locales.
+
 Another common requirement is to host the website on a different domain
 according to the locale. This can be done by defining a different host for each
 locale.