DOC/TST: Add examples to MultiIndex.get_level_values + related changes
- [ ] closes #xxxx
- [x ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I've added examples to ``MultiIndex.get_level_values``. Also I've done some related changes:
* made return value of ``Index._get_level_values``
* Added test for when supplying name to ``Index._get_level_values``. The method could always take level name, but the doc said only integer could be supplied.