docs(document): improve $assertPopulated() docs

lib/document.js

@@ -4417,8 +4417,13 @@ Document.prototype.$populated = Document.prototype.populated;
  *     doc.$assertPopulated('author'); // does not throw
  *     doc.$assertPopulated('other path'); // throws an error
  *
+ *     // Manually populate and assert in one call. The following does
+ *     // `doc.$set({ likes })` before asserting.
+ *     doc.$assertPopulated('likes', { likes });
  *
- * @param {String|String[]} path
+ *
+ * @param {String|String[]} path path or array of paths to check. `$assertPopulated` throws if any of the given paths is not populated.
+ * @param {Object} [values] optional values to `$set()`. Convenient if you want to manually populate a path and assert that the path was populated in 1 call.
  * @return {Document} this
  * @memberOf Document
  * @method $assertPopulated