REF: PandasColumn.describe_categorical returns categores instead of mapping 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

xref: https://github.com/data-apis/dataframe-api/pull/69

cc @shwina @vnlitvinov
@vnlitvinov ok?