DOC: share swaplevel docstring between DataFrame and Series (#42120)
- [ ] closes #42120 
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] tests added / passed
- [ ] whatsnew entry

This commit
- adds examples to the Series.swaplevel docstring
- reuses the same template in Series.swaplevel and DataFrame.swaplevel docstrings

looks fine. @MarcoGorelli can you have a look