diff -u b/core/lib/Drupal/Core/Url.php b/core/lib/Drupal/Core/Url.php
--- b/core/lib/Drupal/Core/Url.php
+++ b/core/lib/Drupal/Core/Url.php
@@ -41,13 +41,46 @@
   use DependencySerializationTrait;
 
   /**
-   * Scheme for base-relative, unrouted URIs.
+   * Scheme for URIs to resolve as references relative to Drupal's installation.
    *
-   * This scheme should only be used for unrouted URIs, like base:robots.txt
-   * or base:core/update.php. To create such a base-relative URI from a path,
-   * use Url::fromBaseRelativePath(). To create a host-relative URI instead
-   * (for example, for http://example.com/index.html when Drupal is installed
-   * in http://example.com/drupal/), use Url::fromHostRelativePath().
+   * https://tools.ietf.org/html/rfc3986#section-5.2 specifies the standard for
+   * resolving URI references relative to a base URI.
+   *
+   * This scheme is to allow such references to be internally represented and
+   * managed in Drupal as full URIs themselves, and resolved to the
+   * corresponding URLs at runtime. When performing this resolution, the base
+   * URI is Drupal's installation URL.
+   *
+   * For example, if Drupal is installed in http://example.com/drupal/, then:
+   * - Url::fromUri(Url::SCHEME_BASE . ':README.txt')->toString() would return
+   *   '/drupal/README.txt'.
+   * - Url::fromUri(Url::SCHEME_BASE . ':/index.html')->toString() would return
+   *   '/index.html'.
+   *
+   * This is the identical resolution as what a browser would perform if Drupal
+   * output a @link http://www.w3.org/wiki/HTML/Elements/base <base href="/drupal/"> @endlink
+   * element and the above URLs were written within links as:
+   * - <a href="README.txt"></a> (browser resolves as '/drupal/README.txt')
+   * - <a href="/index.html"></a> (browser resolves as '/index.html')
+   *
+   * The examples above can also be generated from the wrapper methods
+   * fromBaseRelativePath() and fromHostRelativePath(), as follows:
+   * - Url::fromBaseRelativePath('README.txt') is equivalent to
+   *   Url::fromUri(Url::SCHEME_BASE . ':README.txt').
+   * - Url::fromHostRelativePath('index.html') is equivalent to
+   *   Url::fromUri(Url::SCHEME_BASE . ':/index.html').
+   *
+   * Note that this scheme is intended for linking to non-Drupal-routed pages.
+   * For example, to static files or to pages served by other software on the
+   * website. As such, the resolution of this scheme does not apply any path
+   * processors, such as language prefixing and alias lookup, or access
+   * checking (->access() returns TRUE for URIs in this scheme). Therefore,
+   * using this scheme for Drupal-routed pages is incompatible with
+   * multilingual sites, with search engine optimization, and with hiding links
+   * to pages that the user can't access, unless you take care of all that
+   * yourself. For these reasons, it is recommended to not use this scheme for
+   * pages served by Drupal. The other schemes and their wrapper methods
+   * can be used for that.
    *
    * @see static::fromBaseRelativePath()
    * @see static::fromHostRelativePath()
@@ -253,11 +286,12 @@
    *
    * This method should only be used for paths within the base directory of the
    * site installation that do not have a Drupal route. These may be files like
-   * robots.txt, core scripts like core/update.php, or they may be another
+   * robots.txt, core scripts like core/install.php, or they may be another
    * application placed within the Drupal site installation.
    *
-   * Do not use this method for URLs that do have a Drupal route (such as pages
-   * generated by Drupal) because this will circumvent the routing system.
+   * Do not use this method for URLs that have a Drupal route (that is, most
+   * pages generated by Drupal). See the documentation of static::SCHEME_BASE
+   * for details.
    *
    * @param string $path
    *   The base-relative path, for example 'robots.txt'.
@@ -265,9 +299,10 @@
    *   (optional) An array of options. See static::fromUri() for details.
    *
    * @return \Drupal\Core\Url
-   *   A new Url object for an unrouted, base-relative URL not internal to
-   *   Drupal, but within the site installation directory.
+   *   A new Url object for an unrouted URL within the site installation
+   *   directory.
    *
+   * @see static::SCHEME_BASE
    * @see static::fromHostRelativePath()
    * @see static::fromUri()
    * @see static::fromRoute()
@@ -282,8 +317,9 @@
    * This method should only be used for paths within the same host as the
    * site installation.
    *
-   * Do not use this method for URLs that do have a Drupal route (that is, most
-   * pages generated by Drupal) because this will circumvent the routing system.
+   * Do not use this method for URLs that have a Drupal route (that is, most
+   * pages generated by Drupal). See the documentation of static::SCHEME_BASE
+   * for details.
    *
    * @param string $path
    *   The host-relative path. For example, if Drupal is installed in the
@@ -296,6 +332,7 @@
    *   A new Url object for an unrouted, host-relative URL not internal to
    *   Drupal, but relative to the site host.
    *
+   * @see static::SCHEME_BASE
    * @see static::fromBaseRelativePath()
    * @see static::fromUri()
    * @see static::fromRoute()