GH38482 : added warning in docs to show -1 will be returned for not found values
- [ ] closes #38482
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew: added warning in docs to show -1 will be returned for not found values, with a suitable example.
