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)

