DOC: More examples  comparison with sql
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

added the following examples:
- Top N rows with offset
- Top N rows per group
- ROW_NUMBER() OVER(PARTITION BY day ORDER BY total_bill DESC)
- UPDATE
- DELETE
