docs: add comment for _transformDuplicateKeyError

Author: vkarpov15

lib/schema.js

@@ -2799,8 +2799,13 @@ Schema.prototype._getPathType = function(path) {
   return search(path.split('.'), _this);
 };
 
-/*!
+/**
+ * Transforms the duplicate key error by checking for duplicate key error messages by path.
+ * If no duplicate key error messages are found, returns the original error.
  *
+ * @param {Error} error The error to transform
+ * @returns {Error} The transformed error
+ * @api private
  */
 
 Schema.prototype._transformDuplicateKeyError = function _transformDuplicateKeyError(error) {