BUG: .describe lost CategoricalIndex info
- [x] closes #11558
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry
#11558 is partially fixed by #12531, this PR let `.describe()` to preserve `CategoricalIndex` info. And added explicit test cases derived from #11558.
