Issue #2147899 by anavarre: Better indicate what to fill in the single...

Issue #2147899 by anavarre: Better indicate what to fill in the single import's Configuration name field.

core/modules/config/lib/Drupal/config/Form/ConfigSingleImportForm.php

@@ -139,6 +139,7 @@ public function buildForm(array $form, array &$form_state) {
     );
     $form['config_name'] = array(
       '#title' => $this->t('Configuration name'),
+      '#description' => $this->t('Enter the name of the configuration file without the <em>.yml</em> extension. (e.g. <em>system.site</em>)'),
       '#type' => 'textfield',
       '#states' => array(
         'required' => array(