diff -u b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListItemBase.php b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListItemBase.php
--- b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListItemBase.php
+++ b/core/modules/options/lib/Drupal/options/Plugin/Field/FieldType/ListItemBase.php
@@ -134,13 +134,15 @@
       // case of an alter changing #parents.
       $element['allowed_values']['#on_parents'] = &$element['allowed_values']['on']['#parents'];
       $element['allowed_values']['#off_parents'] = &$element['allowed_values']['off']['#parents'];
+
+      // Provide additional information about how to format allowed_values
+      // of a boolean field for use by a single on/off checkbox widget. Since
+      // the widget might not have been selected yet, can be changed independently
+      // of this form, and can vary by form mode, we display this information
+      // regardless of current widget selection.
+      $element['allowed_values']['#description'] .= '<p>' . t("For a 'single on/off checkbox' widget, define the 'off' value first, then the 'on' value in the <strong>Allowed values</strong> section. Note that the checkbox will be labeled with the label of the 'on' value.") . '</p>';
     }
 
-    // It is no longer possible to alter the description of allowed values
-    // depending on widget type because of form displays. The help text must
-    // instead be verbose.
-    $element['allowed_values']['#description'] .= '<p>' . t("For a 'single on/off checkbox' widget, define the 'off' value first, then the 'on' value in the <strong>Allowed values</strong> section. Note that the checkbox will be labeled with the label of the 'on' value.") . '</p>';
-    $element['allowed_values']['#description'] .= '<p>' . t("The 'checkboxes/radio buttons' widget will display checkboxes if the <em>Number of values</em> option is greater than 1 for this field, otherwise radios will be displayed.") . '</p>';
     $element['allowed_values']['#description'] .= '<p>' . t('Allowed HTML tags in labels: @tags', array('@tags' => _field_filter_xss_display_allowed_tags())) . '</p>';
 
     $element['allowed_values_function'] = array(