DOC: Update the pandas.Index.is_ docstring
- [x] closes https://github.com/pandanistas/pandanistas_sprint_ui2020/issues/6
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Output of `python scripts/validate_docstrings.py pandas.Index.is_`:
```
################################################################################
################################## Validation ##################################
################################################################################

1 Errors found:
        No examples section found
```
@garyteofanus can you see if @WillAyd's suggestion fixes the CI? Closing as stale. @garyteofanus let us know if you finally want to finish this. I'll submit a new pull request for this today as part of PyData Amsterdam sprint.