ENH: truncate output of Groupby.groups
Resurrection of #24853. Have resolved conflicts and updated according to the last review it received. The original PR fixed some unrelated minor typos, so I've kept them in (and added a fix for another absolutely minor typo I'd noticed)

- [x] closes #1135
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
