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.

Hello @shuaggar-sys! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-02-04 18:05:51 UTC on running the below i get stale output, i ran it before i added the first commit, the message is not updated in the generated html file.
`python make.py --single pandas.Index.get_indexer` did you do `python make.py clean`? Sorry for the rookie mistake, i did not do that. Yes everything looks fine after the build.


![pandas_pr_ss](https://user-images.githubusercontent.com/67254493/106975072-d8bf8300-677b-11eb-9155-c163c12e424a.png)
 This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. closing as stale,  please open a new PR if interested in continuing