diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index a51320d..35341ce 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -124,21 +124,33 @@ class DrupalKernel implements DrupalKernelInterface, TerminableInterface {
   protected $containerNeedsDumping;
 
   /**
-   * Holds the list of YAML files containing service definitions.
+   * List of discovered services.yml pathnames.
+   *
+   * This is a nested array whose top-level keys are 'app' and 'site', denoting
+   * the origin of a service provider. Site-specific providers have to be
+   * collected separately, because they need to be processed last, so as to be
+   * able to override services from application service providers.
    *
    * @var array
    */
   protected $serviceYamls;
 
   /**
-   * The list of the classnames of the service providers in this kernel.
+   * List of discovered service provider class names.
+   *
+   * This is a nested array whose top-level keys are 'app' and 'site', denoting
+   * the origin of a service provider. Site-specific providers have to be
+   * collected separately, because they need to be processed last, so as to be
+   * able to override services from application service providers.
    *
    * @var array
    */
   protected $serviceProviderClasses;
 
   /**
-   * The array of registered service providers.
+   * List of instantiated service provider classes.
+   *
+   * @see \Drupal\Core\DrupalKernel::$serviceProviderClasses
    *
    * @var array
    */
@@ -576,7 +588,10 @@ protected function buildContainer() {
    */
   protected function initializeServiceProviders() {
     $this->discoverServiceProviders();
-    $this->serviceProviders = array();
+    $this->serviceProviders = array(
+      'app' => array(),
+      'site' => array(),
+    );
     foreach ($this->serviceProviderClasses as $origin => $classes) {
       foreach ($classes as $name => $class) {
         $this->serviceProviders[$origin][$name] = new $class;
diff --git a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
index a5d76cb..2384497 100644
--- a/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/DrupalKernel/DrupalKernelSiteTest.php
@@ -14,6 +14,9 @@
  */
 class DrupalKernelSiteTest extends DrupalUnitTestBase {
 
+  /**
+   * {@inheritdoc}
+   */
   public static function getInfo() {
     return array(
       'name' => 'DrupalKernel site service overrides',
@@ -25,7 +28,7 @@ public static function getInfo() {
   /**
    * Tests services.yml in site directory.
    */
-  function testServicesYml() {
+  public function testServicesYml() {
     // A service provider class always has precedence over services.yml files.
     // The most common swappable core services are overridden by
     // \Drupal\simpletest\TestServiceProvider already (which proxies into
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
index b2ce930..8671ca6 100644
--- a/sites/default/default.settings.php
+++ b/sites/default/default.settings.php
@@ -49,6 +49,11 @@
  *
  * @see example.sites.php
  * @see conf_path()
+ *
+ * In addition to customizing application settings through variables in
+ * settings.php, you can create a services.yml file in the same directory to
+ * register custom, site-specific service definitions and/or swap out default
+ * implementations with custom ones.
  */
 
 /**