Docstring for pd.core.window.Expanding.kurt
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Notice that I made two small changes to the ``Rolling.kurt``(by @WillAyd ) docstring too.

Namely:
- since window methods have ``kurt`` but not ``kurtosis`` (while e.g. ``Series`` has both), I guess it is best to tell users that ``kurt`` is the default in pandas - that is, use it in "See also"
- it is true that a ``DataFrame`` can be constructed directly from a list, but this is not the standard use, so I replaced with ``Series``