DOC: add note about the values of unit for pd.to_datetime
The `unit` option for `pandas.to_datetime` takes different format strings than the `format` option,
which makes sense, but caught me off-guard as a new user. It would be helpful if the documentation
mentioned this.

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