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``
Thanks @toobaz. The merged update was actually incomplete so I have some conflicting work in #20044 which amongst other things accounts for your second point. I didn't think of your first point though. 

Probably easiest if I just include as part of mine and we close this out, otherwise we'll be doing double work to rebase. OK with that? > Probably easiest if I just include as part of mine and we close this out, otherwise we'll be doing double work to rebase. OK with that?

Sure, just ping when done. Feel free to also include the "See also" descriptions (my last commit). I mean: as long as @datapythonista is OK (level unlocked) ;-) lgtm :) @toobaz pushed a new commit in #20044 integrating a few things of what you had. Give it a look and let me know what you think # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=h1) Report
> Merging [#20064](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/c730d08eca4c7f4455d0fc61802bb712db07aea7?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20064/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20064      +/-   ##
==========================================
+ Coverage   91.72%   91.72%   +<.01%     
==========================================
  Files         150      150              
  Lines       49112    49116       +4     
==========================================
+ Hits        45047    45051       +4     
  Misses       4065     4065
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.1% <100%> (ø)` | :arrow_up: |
| #single | `41.87% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20064/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.33% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=footer). Last update [c730d08...94ed47b](https://codecov.io/gh/pandas-dev/pandas/pull/20064?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 OK, I will rebase this on #20044 when merged, and update. ready to merge, as far as I can tell lgtm. will be merged after sprints.