DOC: DataFrame.reindex improve summary and return type
- [x] closes #33888 
- [x] tests added / passed (passes `scripts/validate_docstrings.py pandas.DataFrame.reindex`)
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry (Not Required)