DOC: Add summary to interpolate
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

The interpolate method was missing a short summary. I hope this addition is reasonable. Happy to change it with feedback.
Thanks @erictleung 