DOC: Move notes to appropriate section in agg and aggregate docs
- [x] closes #52253

Move a paragraph of notes from `Returns` to `Notes` in the docs of `pandas.DataFrame.agg` and `pandas.DataFrame.aggregate`, which removes the incorrect bolding.
/preview Website preview of this PR available at: https://pandas.pydata.org/preview/52267/ thx @futureprogrammer360 