DOC: Clarify relation between DateOffset and relativedelta
dateutil.relativedelta can be constructed either from two date/datetime objects passed as positional arguments, or explicitly constructed in terms of year, month, day etc passed as keyword arguments. DateOffset only supports the keyword argument form of relativedelta.

- [x] related to #41847
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

Thanks @daviewales 