Issue #2474469 by Mac_Weber, rootwork, xjm: Add help text to the option "internal links only"

core/modules/link/src/Plugin/Field/FieldWidget/LinkWidget.php

@@ -193,9 +193,10 @@ public function formElement(FieldItemListInterface $items, $delta, array $elemen
     }
 
     // If the field is configured to allow only internal links, add a useful
-    // element prefix.
+    // element prefix and description.
     if (!$this->supportsExternalLinks()) {
       $element['uri']['#field_prefix'] = rtrim(\Drupal::url('<front>', array(), array('absolute' => TRUE)), '/');
+      $element['uri']['#description'] = $this->t('This must be an internal path such as %add-node. You can also start typing the title of a piece of content to select it. Enter %front to link to the front page.', array('%add-node' => '/node/add', '%front' => '<front>'));
     }
     // If the field is configured to allow both internal and external links,
     // show a useful description.