code: def _dir_additions(self):
          
          additions = {c for c in self._info_axis.unique(level=0)[:100]
                       if isinstance(c, str) and c.isidentifier()}
          return super()._dir_additions().union(additions)
docs: add the string-like attributes from the info_axis.
          If info_axis is a MultiIndex, it's first level values are used.