Fix DataFrame/Series stack/unstack docs
- [x] closes #21675
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

As discussed in the issue, the sorted level is an implementation detail that does not always happen. Thus I think we should remove the misleading documentation.