docs: def validate_boolean_or_string(option, value):
      """Validates that value is True, False, 'true', or 'false'."""
      if isinstance(value, string_type):
          if value not in ('true', 'false'):
              raise ValueError("The value of %s must be "
                               "'true' or 'false'" % (option,))
          return value == 'true'
      return validate_boolean(option, value)