DOC: added examples to DataFrame.var #44162
- [x] closes #44162 by adding two examples to docstring of DataFrame.var
- [x] tests added / passed
- [x] All linting tests passed (pre-commit)

Screenshot of locally built documentation:

![var](https://user-images.githubusercontent.com/58558211/139727701-f6c92ad1-948d-42bd-8989-2d874e3cfa58.PNG)


