Show display plugin descriptions on the embed button form.

src/Annotation/EntityEmbedDisplay.php

@@ -32,7 +32,16 @@ class EntityEmbedDisplay extends Plugin {
    *
    * @var \Drupal\Core\Annotation\Translation
    */
-  public $label = '';
+  public $label;
+
+  /**
+   * A short description of the display plugin.
+   *
+   * @ingroup plugin_translatable
+   *
+   * @var \Drupal\Core\Annotation\Translation
+   */
+  public $description;
 
   /**
    * The entity types the display can apply to.

src/Plugin/Derivative/FieldFormatterDeriver.php

@@ -57,9 +57,13 @@ public function getDerivativeDefinitions($base_plugin_definition) {
     if (!isset($base_plugin_definition['field_type'])) {
       throw new \LogicException("Undefined field_type definition in plugin {$base_plugin_definition['id']}.");
     }
+    $plugin_definitions = $this->formatterManager->getDefinitions();
     foreach ($this->formatterManager->getOptions($base_plugin_definition['field_type']) as $formatter => $label) {
       $this->derivatives[$formatter] = $base_plugin_definition;
       $this->derivatives[$formatter]['label'] = $label;
+      if (isset($plugin_definitions[$formatter]['description'])) {
+        $this->derivatives[$formatter]['description'] = $plugin_definitions[$formatter]['description'];
+      }
     }
     return $this->derivatives;
   }

src/Plugin/EmbedType/Entity.php

@@ -118,6 +118,12 @@ public function buildConfigurationForm(array $form, FormStateInterface $form_sta
         '#description' => $this->t('If none are selected, all are allowed. Note that these are the plugins which are allowed for this entity type, all of these might not be available for the selected entity.'),
       );
       $form['display_plugins']['#access'] = !empty($form['display_plugins']['#options']);
+      $display_plugins = $this->displayPluginManager->getDefinitions();
+      foreach (array_keys($form['display_plugins']['#options']) as $plugin_id) {
+        if (!empty($display_plugins[$plugin_id]['description'])) {
+          $form['display_plugins'][$plugin_id]['#description'] = $display_plugins[$plugin_id]['description'];
+        }
+      }
     }
 
     return $form;