diff --git a/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
index 999f447..410dd1b 100644
--- a/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -44,7 +44,7 @@ class Url {
   protected $accessManager;
 
   /**
-   * The route name.
+   * The route name or the unrouted URI.
    *
    * @var string
    */
@@ -102,9 +102,10 @@ class Url {
    * code more self-documenting.
    *
    * @param string $route_name
-   *   The name of the route
+   *   The name of the route or an URI without a Drupal route.
    * @param array $route_parameters
-   *   (optional) An associative array of parameter names and values.
+   *   (optional) An associative array of parameter names and values for the
+   *   route. This is not used if $route_name is an unrouted URI.
    * @param array $options
    *   (optional) An associative array of additional options, with the following
    *   elements: