Return the indices that would sort the Categorical.

        Missing values are sorted at the end.

        Parameters
        ----------
        ascending : bool, default True
            Whether the indices should result in an ascending
            or descending sort.
        kind : {'quicksort', 'mergesort', 'heapsort', 'stable'}, optional
            Sorting algorithm.
        **kwargs:
            passed through to :func:`numpy.argsort`.

        Returns
        -------
        np.ndarray[np.intp]

        See Also
        --------
        numpy.ndarray.argsort

        Notes
        -----
        While an ordering is applied to the category values, arg-sorting
        in this context refers more to organizing and grouping together
        based on matching category values. Thus, this function can be
        called on an unordered Categorical instance unlike the functions
        'Categorical.min' and 'Categorical.max'.

        Examples
        --------
        >>> pd.Categorical(['b', 'b', 'a', 'c']).argsort()
        array([2, 0, 1, 3])

        >>> cat = pd.Categorical(['b', 'b', 'a', 'c'],
        ...                      categories=['c', 'b', 'a'],
        ...                      ordered=True)
        >>> cat.argsort()
        array([3, 0, 1, 2])

        Missing values are placed at the end

        >>> cat = pd.Categorical([2, None, 1])
        >>> cat.argsort()
        array([2, 0, 1])
        r.