DOC add extended summary, update parameter types desc, update return types desc, and add whitespaces after commas in list declarations to DataFrame.first in core/generic.py
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

@sagungrp I think CI failure is unrelated but can you merge master and repush to be sure? @sagungrp can you please merge master into your branch to fix the CI error?
```
git fetch upstream
git merge upstream/master
git push
```
This is ready, we can merge after that. Thanks! Thanks @sagungrp 