DOC: Added sentence to show that unmatched values return -1.
- [x] closes #38482
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [x] whatsnew: Added sentence to show that unmatched values return -1 in get_indexer method

In [1]: python make.py --single pandas.Index.get_indexer
Out [1]:![image](https://user-images.githubusercontent.com/50010452/134762263-3be88054-ddfa-4fee-889c-112c00cf780e.png)

In [2]: pre-commit run --files pandas/core/indexes/base.py
Out [2] : ![image](https://user-images.githubusercontent.com/50010452/134768894-a75b8ade-175e-477d-94ff-fefa224f98f9.png)


Hello @DhruvBShetty! 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 2021-11-01 12:05:03 UTC 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. Hi @DhruvBShetty can you address the comment please > 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.

Interested in finishing this. > Can you run the [validating docstrings](https://pandas.pydata.org/docs/development/contributing_documentation.html#updating-a-pandas-docstring) script for this function?

Done, It's the first error
![image](https://user-images.githubusercontent.com/50010452/139662439-9a7d1f8c-5d3e-4e0d-8657-1ad0e45eaae5.png)

After which it's this
![image](https://user-images.githubusercontent.com/50010452/139662605-d267c753-630b-426d-b46d-5e7ea5a9ff39.png)

Left a line between %(raises_section)s and notes accidentally.
![image](https://user-images.githubusercontent.com/50010452/139662878-b94ea7d9-aefc-4e61-8362-3045f8c18d81.png)


 Thanks @DhruvBShetty 