code: func (s *SearchSource) DocvalueFields(docvalueFields ...string) *SearchSource {
  	for _, f := range docvalueFields {
  		s.docvalueFields = append(s.docvalueFields, DocvalueField{Field: f})
  	}
  	return s
  }
docs: // DocvalueFields adds one or more fields to load from the field data cache
  // and return as part of the search request.