diff --git a/core/modules/system/core.api.php b/core/modules/system/core.api.php
index ad73fa0..0f5aec0 100644
--- a/core/modules/system/core.api.php
+++ b/core/modules/system/core.api.php
@@ -989,10 +989,11 @@
 /**
  * @defgroup extending Extending and altering Drupal
  * @{
- * Overview of add-ons and alteration methods for Drupal.
+ * Overview of extensions and alteration methods for Drupal.
  *
+ * @section sec_types Types of extensions
  * Drupal's core behavior can be extended and altered via these three basic
- * types of add-ons:
+ * types of extensions:
  * - Themes: Themes alter the appearance of Drupal sites. They can include
  *   template files, which alter the HTML markup and other raw output of the
  *   site; CSS files, which alter the styling applied to the HTML; and
@@ -1005,8 +1006,9 @@
  * - Installation profiles: Installation profiles can be used to
  *   create distributions, which are complete specific-purpose packages of
  *   Drupal including additional modules, themes, and data. For more
- *   information, see https://drupal.org/documentation/build/distributions.
+ *   information, see https://www.drupal.org/node/159730.
  *
+ * @section sec_alter Alteration methods for modules
  * Here is a list of the ways that modules can alter or extend Drupal's core
  * behavior, or the behavior of other modules:
  * - Hooks: Specially-named functions that a module defines, which are
@@ -1025,6 +1027,33 @@
  * - Routing: Providing or altering "routes", which are URLs that Drupal
  *   responds to, or altering routing behavior with event listener classes.
  *   See the @link menu Routing and menu topic @endlink for more information.
+ *
+ * @section sec_sample *.info.yml files
+ * Extensions must each be located in a directory whose name matches the short name
+ * (or machine name) of the extension, and this directory must contain a file named
+ * machine_name.info.yml (where machine_name is the machine name of the extension).
+ * The syntax of the file is similar for modules, themes, and installation profiles.
+ * Here is a sample (for a module):
+ * @code
+ * # Human-readable name of the extension, shown in the UI.
+ * name: 'My Example'
+ * # Text that is shown on the module administration page.
+ * description: 'Description of what this module does.'
+ * # This key is used to group related modules together in the UI.
+ * package: Custom
+ * # Type of extension: module, theme, or profile.
+ * type: module
+ * # For custom modules, define a version number. If your extension is on drupal.org,
+ * # use version: VERSION and the packaging scripts will insert the version.
+ * version: VERSION
+ * # Specifies the version of Drupal core that your module is compatible with.
+ * core: 8.x
+ * # If your module depends on other modules, list them here.
+ * dependencies:
+ * - node
+ * # Route for the main administration page.
+ * configure: example.admin_index.
+ * @endcode
  * @}
  */