docs: def touch(self, expiration):
          """Updates the current document's expiration value.
  
          :param expiration: Expiration in seconds for the document to be removed by
              couchbase server, defaults to 0 - will never expire.
          :type expiration: int
          :returns: Response from CouchbaseClient.
          :rtype: unicode
          :raises: :exc:'cbwrapper.errors.DoesNotExist' or
              :exc:'couchbase.exception.TemporaryFailError'
          """
          if not self.cas_value or not self.doc_id:
              raise self.DoesNotExist(self)
          return self.bucket.touch(self.doc_id, expiration)