code: def document_permissions(self):
          
          if self._document_permissions is None:
              self._document_permissions = DocumentPermissionList(
                  self._version,
                  service_sid=self._solution['service_sid'],
                  document_sid=self._solution['sid'],
              )
          return self._document_permissions
docs: Access the document_permissions
  
          :returns: twilio.rest.preview.sync.service.document.document_permission.DocumentPermissionList
          :rtype: twilio.rest.preview.sync.service.document.document_permission.DocumentPermissionList