docs: def value(self):
          """Property to be used when saving a custom field into
          :class:`couchbasekit.document.Document` instance.
  
          :returns: The value to be saved for the field within
              :class:`couchbasekit.document.Document` instances.
          :rtype: mixed
          """
          if self._value is None:
              raise ValueError("%s's 'value' is not set." % type(self).__name__)
          return self._value