DOC: Update the pandas.core.window.x.sum docstring
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 "```" 
(pandas-dev) D:\GitHub\pandas>python scripts/validate_docstrings.py pandas.core.window.Rolling.sum

################################################################################
################## Docstring (pandas.core.window.Rolling.sum) ##################
################################################################################

Calculate rolling sum of given DataFrame or Series.

Parameters
----------
*args
    Under Review.
**kwargs
    Under Review.

Returns
-------
Series or DataFrame
    Like-indexed object containing the result of function application

See Also
--------
Series.rolling : Calling object with Series data
DataFrame.rolling  : Calling object with DataFrame data
Series.sum  : Equivalent method for Series
DataFrame.sum   : Equivalent method for DataFrame

Examples
--------
>>> s = pd.Series([1, 2, 3, 4, 5])
>>> s.rolling(3).sum()
0     NaN
1     NaN
2     6.0
3     9.0
4    12.0
dtype: float64
>>> s.expanding(3).sum()
0     NaN
1     NaN
2     6.0
3    10.0
4    15.0
dtype: float64
>>> s.rolling(3, center=True).sum()
0     NaN
1     6.0
2     9.0
3    12.0
4     NaN
dtype: float64

################################################################################
################################## Validation ##################################
################################################################################

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

(pandas-dev) D:\GitHub\pandas>python scripts/validate_docstrings.py pandas.core.window.Expanding.sum

################################################################################
################# Docstring (pandas.core.window.Expanding.sum) #################
################################################################################

Calculate expanding sum of given DataFrame or Series.

Parameters
----------
*args
    Under Review.
**kwargs
    Under Review.

Returns
-------
Series or DataFrame
    Like-indexed object containing the result of function application

See Also
--------
Series.expanding : Calling object with Series data
DataFrame.expanding  : Calling object with DataFrame data
Series.sum  : Equivalent method for Series
DataFrame.sum   : Equivalent method for DataFrame

Examples
--------
>>> s = pd.Series([1, 2, 3, 4, 5])
>>> s.rolling(3).sum()
0     NaN
1     NaN
2     6.0
3     9.0
4    12.0
dtype: float64
>>> s.expanding(3).sum()
0     NaN
1     NaN
2     6.0
3    10.0
4    15.0
dtype: float64
>>> s.rolling(3, center=True).sum()
0     NaN
1     6.0
2     9.0
3    12.0
4     NaN
dtype: float64

################################################################################
################################## Validation ##################################
################################################################################

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

(pandas-dev) D:\GitHub\pandas>python scripts/validate_docstrings.py pandas.core.window.Window.sum

################################################################################
################## Docstring (pandas.core.window.Window.sum)  ##################
################################################################################

Calculate window sum of given DataFrame or Series.

Parameters
----------
*args
    Under Review.
**kwargs
    Under Review.

Returns
-------
Series or DataFrame
    Like-indexed object containing the result of function application

See Also
--------
Series.window : Calling object with Series data
DataFrame.window  : Calling object with DataFrame data
Series.sum  : Equivalent method for Series
DataFrame.sum   : Equivalent method for DataFrame

Examples
--------
>>> s = pd.Series([1, 2, 3, 4, 5])
>>> s.rolling(3).sum()
0     NaN
1     NaN
2     6.0
3     9.0
4    12.0
dtype: float64
>>> s.expanding(3).sum()
0     NaN
1     NaN
2     6.0
3    10.0
4    15.0
dtype: float64
>>> s.rolling(3, center=True).sum()
0     NaN
1     6.0
2     9.0
3    12.0
4     NaN
dtype: float64

################################################################################
################################## Validation ##################################
################################################################################

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


