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

k looks reasonable thanks @aidanmontare-edu  No problem!

On Sat, Jun 20, 2020 at 4:12 PM Jeff Reback <notifications@github.com>
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-- 
Sincerely,

Aidan Montare
CWRU Class of 2021
