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">

Hello @onshek! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2020-08-29 07:56:56 UTC Sorry for closing it by mistake :) Can anyone help me to figure out why two new checks failed here (This PR passed all 15/15 checks two days ago and I didn't make new changes)? Thanks! @MarcoGorelli @charlesdong1991 @rhshadrach  > Can anyone help me to figure out why two new checks failed here (This PR passed all 15/15 checks two days ago and I didn't make new changes)? Thanks! @MarcoGorelli @charlesdong1991 @rhshadrach

Failures are unrelated, if you merge master and push again I'd expect them to be fixed > > Can anyone help me to figure out why two new checks failed here (This PR passed all 15/15 checks two days ago and I didn't make new changes)? Thanks! @MarcoGorelli @charlesdong1991 @rhshadrach
> 
> Failures are unrelated, if you merge master and push again I'd expect them to be fixed

Got it, thanks! Thanks @onshek. The CI failure is unrelated
```

============================= test session starts ==============================
platform linux -- Python 3.8.5, pytest-6.0.1, py-1.9.0, pluggy-0.13.1
rootdir: /home/vsts/work/1/s, configfile: setup.cfg, testpaths: pandas
plugins: hypothesis-5.28.0, forked-1.2.0, asyncio-0.12.0, xdist-2.1.0, azurepipelines-0.8.0
gw0 I / gw1 I
gw0 [76009] / gw1 [76009]

##[error]1 test(s) failed, 76009 test(s) collected.
Skipping uploading of coverage data.

=================================== FAILURES ===================================
_____________________ TestIndex.test_tab_complete_warning ______________________

```