Issue #3339837: Implement hook_help()

preview.module

@@ -6,6 +6,25 @@
  */
 
 use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Implements hook_help().
+ */
+function all_entity_preview_help($route_name, RouteMatchInterface $route_match) {
+  switch ($route_name) {
+    // Main module help for the all_entity_preview module.
+    case 'help.page.all_entity_preview':
+      $output = '';
+      $output .= '<h3>' . t('About') . '</h3>';
+      $output .= '<p>' . t('This module allows to set an optional preview on any
+      content entity type and bundle as its currently implemented for nodes. The
+      code is based on the one in the core Node module.') . '</p>';
+      return $output;
+
+    default:
+  }
+}
 /**
  * Implements hook_page_top().
  */