DOC: Add details to DataFrame groupby transform
- [X] closes #13543
- [X] tests added / passed
- [X] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

Add requirements for user function in groupby transform

closes #13543
[skip ci]

@jorisvandenbossche I've taken another stab at explaining the requirements.  Also updated the requirements in the online docs.
 also would like to update the doc-string with the same @bashtage can you rebase / update I've tried again to explain.  Maybe it would be safer to just say "Don't use f that mutate the data or make use of data in multiple columns to compute the transformation"? @bashtage can you update thanks @bashtage I added a templated version to handle SeriesGroupBy as well