Clarify docs for df.to_sql `chunksize`
- [x] closes #35891
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Note: this is a docs pr, so not adding tests or a whatsnew entry. 
Thanks for the PR but looks like more discussion is needed in original issue before doing anything here