clarified the documentation for DF.drop_duplicates
I hit an issue with a time series index where I wanted to keep duplicate data with different time values and only delete rows with the same time and columns. This documentation change would have saved me a lot of time.

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