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`

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<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">
