DOC: Fix groupby.agg/transform API reference and add numba arguments to documentation
- xref https://github.com/pandas-dev/pandas/pull/33388#issuecomment-618808443
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

<img width="927" alt="Screen Shot 2020-04-27 at 11 19 16 PM" src="https://user-images.githubusercontent.com/10647082/80453833-e20eeb80-88dd-11ea-9acc-ef322cb59df5.png">
<img width="934" alt="Screen Shot 2020-04-27 at 11 19 33 PM" src="https://user-images.githubusercontent.com/10647082/80453838-e4714580-88dd-11ea-85b2-735d327ee686.png">
<img width="922" alt="Screen Shot 2020-04-27 at 11 19 53 PM" src="https://user-images.githubusercontent.com/10647082/80453844-e6d39f80-88dd-11ea-8c49-a9d35fd80e0c.png">
<img width="927" alt="Screen Shot 2020-04-27 at 11 20 16 PM" src="https://user-images.githubusercontent.com/10647082/80453851-e935f980-88dd-11ea-8c62-b92043ed26e4.png">

Hello @mroeschke! 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 2020-04-28 17:18:40 UTC