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
