DOC GH22893 Fix docstring of groupby in pandas/core/generic.py
- [x] closes #22893 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Updated docstring following https://pandas.pydata.org/pandas-docs/stable/contributing_docstring.html

I changed the examples into similar ones that I thought better demonstrated the function, but I'm willing to change them again if they aren't up to par.

I also might need some help describing the kwargs. It only accepts 'mutated' as a keyword argument and I can't tell if it does anything. Apologies if the answer is obvious!
