DOC: `MultiIndex.get_level_values` mentions implicit type casting if level contains null.
- [x] closes #44792 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit) for how to run them
- [ ] whatsnew entry
I was hesitant to write "internally MultiIndex is implemented by codes but regular index doesn't", concerning that reveals too much implementation details in the docstring. Let me know if it fits in. can you add an example which illustrates this as well. Example added. @jreback  thanks @isVoid  @meeseeksdev backport 1.4.x Something went wrong ... Please have a look at my logs.