DOC: timedelta return type
- [x] closes #42316
- [NA] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [NA] whatsnew entry

Thanks @loicdiridollou! Awesome job. Happy to have more of these doc issues addressed! 