DOC: Added example to MultiIndex doc string
- [ ] closes #xxxx
- [ ] tests added / passed
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

I think ``MultiIndex`` doc string should have a quick note on how ``MultiIndex`` is typically instantiated, as you normally wouldn't instantiate it directly.

I've added an example to the doc string, as in my experience  ``MultiIndex`` is one of the more complex areas of Pandas, so a quick example is justified. This may be debatable, so if others disagree on having an example here, I can remove the example and just retain the links to the ``.from_*`` methods.