DOC: Fix groupby.agg/transform rst reference and numba references
- [x] xref https://github.com/pandas-dev/pandas/pull/33388#issuecomment-618808443
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

looks fine the CI/checks buidl is failing