DOC: complement the documentation for pandas.DataFrame.agg #35912
- [x] closes #35912
- [x] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
<img width="676" alt="Screenshot 2020-08-28 at 11 59 23" src="https://user-images.githubusercontent.com/21543236/91519967-a71dd280-e926-11ea-87f5-e647fe650168.png">
