diff --git a/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
index 04a961c..123cc7a 100644
--- a/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -70,7 +70,7 @@ class Url {
   protected $external = FALSE;
 
   /**
-   * Indicates whether this URL is a URI instead of a route.
+   * Indicates whether this URL is for a URI without a Drupal route.
    *
    * @var bool
    */
@@ -88,6 +88,10 @@ class Url {
   /**
    * Constructs a new Url object.
    *
+   * In most cases, use Url::routed() or Url::unrouted rather than constructing
+   * Url objects directly in order to avoid ambiguity and make your code more
+   * self-documenting.
+   *
    * @param string $route_name
    *   The name of the route
    * @param array $route_parameters
@@ -109,6 +113,12 @@ class Url {
    *     defined, the current scheme is used, so the user stays on HTTP or HTTPS
    *     respectively. if mixed mode sessions are permitted, TRUE enforces HTTPS
    *     and FALSE enforces HTTP.
+   *
+   * @see static::routed()
+   * @see static::unrouted()
+   *
+   * @todo Update this documentation for non-routed URIs in
+   *   https://www.drupal.org/node/2346787
    */
   public function __construct($route_name, $route_parameters = array(), $options = array()) {
     $this->routeName = $route_name;
@@ -117,15 +127,19 @@ public function __construct($route_name, $route_parameters = array(), $options =
   }
 
   /**
-   * Creates a new routed (internal to Drupal) Url object.
+   * Creates a new Url object for a URL that has a Drupal route.
+   *
+   * This method is for URLs that have Drupal routes (that is, most pages
+   * generated by Drupal). For non-routed local URIs relative to the base
+   * path (like robots.txt) use Url::unrouted() with the base:// scheme.
    *
    * @param string $route_name
    *   The name of the route
    * @param array $route_parameters
-   *   (optional) An associative array of parameter names and values.
+   *   (optional) An associative array of route parameter names and values.
    * @param array $options
-   *   (optional) An associative array of additional options, with the following
-   *   elements:
+   *   (optional) An associative array of additional URL options, with the
+   *   following elements:
    *   - 'query': An array of query key/value-pairs (without any URL-encoding)
    *     to append to the URL. Merged with the parameters array.
    *   - 'fragment': A fragment identifier (named anchor) to append to the URL.
@@ -142,22 +156,28 @@ public function __construct($route_name, $route_parameters = array(), $options =
    *     and FALSE enforces HTTP.
    *
    * @return \Drupal\Core\Url
-   *   A new routed (internal to Drupal) Url object.
+   *   A new Url object for a routed (internal to Drupal) URL.
+   *
+   * @see static::unrouted()
    */
   public static function routed($route_name, $route_parameters = array(), $options = array()) {
     return new static($route_name, $route_parameters, $options);
   }
 
   /**
-   * Creates a new unrouted (external to Drupal) Url object.
+   * Creates a new Url object for a URI that does not have a Drupal route.
+   *
+   * This method is for generating URLs for URIs that do not have Drupal
+   * routes, both external URLs and unrouted local URIs like
+   * base://robots.txt. For URLs that have Drupal routes (that is, most pages
+   * generated by Drupal), use Url::routed().
    *
    * @param string $uri
    *   The URI of the external resource including the scheme. For Drupal paths
-   *   that are not handled by the routing system, you may use base:// for the
-   *   scheme.
+   *   that are not handled by the routing system, use base:// for the scheme.
    * @param array $options
-   *   (optional) An associative array of additional options, with the following
-   *   elements:
+   *   (optional) An associative array of additional URL options, with the
+   *   following elements:
    *   - 'query': An array of query key/value-pairs (without any URL-encoding)
    *     to append to the URL. Merged with the parameters array.
    *   - 'fragment': A fragment identifier (named anchor) to append to the URL.
@@ -174,7 +194,9 @@ public static function routed($route_name, $route_parameters = array(), $options
    *     and FALSE enforces HTTP.
    *
    * @return \Drupal\Core\Url
-   *   A new unrouted (external to Drupal) Url object.
+   *   A new Url object for an unrouted (non-Drupal) URL.
+   *
+   * @see static::routed()
    */
   public static function unrouted($uri, $options = array()) {
     $url = new static($uri, array(), $options);
@@ -221,10 +243,13 @@ public static function createFromRequest(Request $request) {
   protected function setUnrouted() {
     $this->unrouted = TRUE;
     // What was passed in as the route name is actually the URI.
+    // @todo Consider fixing this in https://www.drupal.org/node/2346787.
     $this->uri = $this->routeName;
     // Set empty route name and parameters.
     $this->routeName = NULL;
     $this->routeParameters = array();
+    // @todo Add a method for the check below in
+    // https://www.drupal.org/node/2346859.
     if ($this->external = strpos($this->uri, 'base://') !== 0) {
       $this->options['external'] = TRUE;
     }
@@ -383,11 +408,11 @@ public function setOption($name, $value) {
    *   A URI not connected to a route. May be an external URL.
    *
    * @throws \UnexpectedValueException
-   *   Thrown when the uri was requested for route.
+   *   Thrown when the URI was requested for a routed URL.
    */
   public function getUri() {
     if (!$this->unrouted) {
-      throw new \UnexpectedValueException('Internal routes do not have URIs.');
+      throw new \UnexpectedValueException('This URL has a Drupal route, so the canonical form is not a URI.');
     }
 
     return $this->uri;