Added Documentation to specify that DataFrame.last() needs the index to be sorted to deliver the expected results
Added Documentation mentioning that DataFrame.last() needs the index to be sorted to deliver the expected results

Haven't yet worked on raising an error will work as advised

- [ ] closes #38000
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @aflah02! 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 2020-12-17 08:21:54 UTC Looks good, thanks @aflah02 (and thanks @arw2019 for reviewing!) Thanks @MarcoGorelli 