Issue #3195743 by dieterholvoet: Improve documentation on how to create a variation

http_cache_control.module

@@ -7,6 +7,7 @@
 
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\Site\Settings;
 
 /**
  * Implements hook_help().
@@ -225,9 +226,13 @@ function http_cache_control_form_system_performance_settings_alter(&$form, FormS
     '#type' => 'textfield',
     '#title' => t('Header variation'),
     '#default_value' => $config->get('cache.http.vary'),
-    '#description' => t('Comma separated list of headers to vary cache globally.'),
+    '#description' => t('A comma-separated list of request header names that could have influenced the generation of this response.'),
   ];
 
+  if (!Settings::get('omit_vary_cookie')) {
+    $form['variation']['vary']['#description'] .= ' ' . t("The 'Cookie' header is automatically added.");
+  }
+
   $form['#submit'][] = 'http_cache_control_form_system_performance_settings_submit';
 }