docs: def _make_c_string(string):
      """Make a 'C' string."""
      if isinstance(string, bytes):
          try:
              _utf_8_decode(string, None, True)
              return string + b"\x00"
          except UnicodeError:
              raise InvalidStringData("strings in documents must be valid "
                                      "UTF-8: %r" % string)
      else:
          return _utf_8_encode(string)[0] + b"\x00"