MC-943: Grouping content blocks and containers in the Panel

- add description for interface

Author: anthoula

app/code/Magento/PageBuilder/view/adminhtml/web/ts/js/utils/promise-deferred.d.ts

@@ -4,6 +4,8 @@
  */
 
 /**
+ * Interface for deferred promise
+ *
  * @api
  */
 export default interface DeferredInterface {