DOC: Improve the docstring of pandas.core.window.Rolling.median
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [x] PR title is "DOC: update the <your-function-or-method> docstring"
- [x] The validation script passes: `scripts/validate_docstrings.py <your-function-or-method>`
- [x] The PEP8 style check passes: `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] The html version looks good: `python doc/make.py --single <your-function-or-method>`
- [x] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
# paste output of "scripts/validate_docstrings.py <your-function-or-method>" here
# between the "```" (remove this comment, but keep the "```")

```
Errors found:
        No extended summary found
        Errors in parameters section
                Parameters {'kwargs'} not documented
                Unknown parameters {'**kwargs'}
                Parameter "**kwargs" has no type

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.

Median is pretty straightforward. Maybe an extended summary is not really necessary? Kwargs needs further review.
Thanks for the feedback everyone. Anything else? LGTM, the only think I'm not sure about is what to put under kwargs. Don't know if we really expect any args there, or it's there only for compatibility purposes.

Looking at https://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.rolling.html I believe most arguments you could pass to https://pandas.pydata.org/pandas-docs/version/0.17.0/generated/pandas.rolling_median.html are already in the rolling window definition. But in `pandas.rolling_median` there are also `**kwargs`... 

In the mailing list someone proposed to leave unclear kwargs documented as "Needs Review" and look at them in the future, so this works for me.

@JoanMartin ? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20260?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@fb556ed`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20260/graphs/tree.svg?width=650&height=150&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20260?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #20260   +/-   ##
=========================================
  Coverage          ?   91.73%           
=========================================
  Files             ?      150           
  Lines             ?    49166           
  Branches          ?        0           
=========================================
  Hits              ?    45100           
  Misses            ?     4066           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.11% <ø> (?)` | |
| #single | `41.86% <ø> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20260?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/20260/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.32% <ø> (ø)` | |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20260?src=pr&el=footer). Last update [fb556ed...702abc4](https://codecov.io/gh/pandas-dev/pandas/pull/20260?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @mehemken ! Stupid question because I'm very surprised: How to you add commits to a PR which is based on a personal fork of the project, if you are not the owner of the fork? ok nvm https://help.github.com/articles/allowing-changes-to-a-pull-request-branch-created-from-a-fork/