code: func (s *BulkService) Refresh(refresh string) *BulkService {
  	s.refresh = refresh
  	return s
  }
docs: // Refresh controls when changes made by this request are made visible
  // to search. The allowed values are: "true" (refresh the relevant
  // primary and replica shards immediately), "wait_for" (wait for the
  // changes to be made visible by a refresh before reying), or "false"
  // (no refresh related actions). The default value is "false".
  //
  // See https://www.elastic.co/guide/en/elasticsearch/reference/7.0/docs-refresh.html
  // for details.