Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/help/help.api.php

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+<?php
+
+/**
+ * @file
+ * Hooks for the Help system.
+ */
+
+/**
+ * @addtogroup hooks
+ * @{
+ */
+
+/**
+ * Provide online user help.
+ *
+ * By implementing hook_help(), a module can make documentation available to
+ * the user for the module as a whole, or for specific pages. Help for
+ * developers should usually be provided via function header comments in the
+ * code, or in special API example files.
+ *
+ * The page-specific help information provided by this hook appears in the
+ * Help block (provided by the core Help module), if the block is displayed on
+ * that page. The module overview help information is displayed by the Help
+ * module. It can be accessed from the page at admin/help or from the Extend
+ * Extend page. If a module implements hook_help() the help system expects
+ * module overview help to be provided.
+ *
+ * For detailed usage examples of:
+ * - Module overview help, see content_translation_help(). Module overview
+ *   help should follow
+ *   @link https://www.drupal.org/node/632280 the standard help template. @endlink
+ * - Page-specific help using only routes, see book_help().
+ * - Page-specific help using routes and $request, see block_help().
+ *
+ * @param string $route_name
+ *   For page-specific help, use the route name as identified in the
+ *   module's routing.yml file. For module overview help, the route name
+ *   will be in the form of "help.page.$modulename".
+ * @param Drupal\Core\Routing\RouteMatchInterface $route_match
+ *   The current route match. This can be used to generate different help
+ *   output for different pages that share the same route.
+ *
+ * @return string
+ *   A localized string containing the help text.
+ */
+function hook_help($route_name, \Drupal\Core\Routing\RouteMatchInterface $route_match) {
+  switch ($route_name) {
+    // Main module help for the block module.
+    case 'help.page.block':
+      return '<p>' . t('Blocks are boxes of content rendered into an area, or region, of a web page. The default theme Bartik, for example, implements the regions "Sidebar first", "Sidebar second", "Featured", "Content", "Header", "Footer", etc., and a block may appear in any one of these areas. The <a href="!blocks">blocks administration page</a> provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions.', array('!blocks' => \Drupal::url('block.admin_display'))) . '</p>';
+
+    // Help for another path in the block module.
+    case 'block.admin_display':
+      return '<p>' . t('This page provides a drag-and-drop interface for assigning a block to a region, and for controlling the order of blocks within regions. Since not all themes implement the same regions, or display regions in the same way, blocks are positioned on a per-theme basis. Remember that your changes will not be saved until you click the <em>Save blocks</em> button at the bottom of the page.') . '</p>';
+  }
+}
+
+/**
+ * @} End of "addtogroup hooks".
+ */