DOC: correct Series.searchsorted example
two major changes:
- categories must be ordered using series.searchsorted.
- delete last two exsamples because of `ValueError: Value(s) to be inserted must be in categories`. 

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