DOC: update the Series.dt.strftime docstring

[mircea-cosbuc]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

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

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

################################################################################
#################### Docstring (pandas.Series.dt.strftime)  ####################
################################################################################

Format series values with a date format string.

This function formats series values as specified by `date_format`, which
supports the same string format as the python standard library. Details
of the string format can be found in `python string format doc <https://docs.python.org/3/library/datetime.html#strftime-and-strptime-behavior>`__.

Parameters
----------
date_format : str
    Date format string (e.g. "%Y-%m-%d").

Returns
-------
ndarray of formatted strings

Examples
--------
>>> rng = pd.date_range('10/3/2018 16:20:10', periods=2, freq='H')
>>> rng.strftime("%Y-%m-%d")
array(['2018-10-03', '2018-10-03'], dtype='<U10')

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

Errors found:
        Errors in parameters section
                Parameters {'kwargs', 'args'} not documented
                Unknown parameters {'date_format'}
        See Also section not found

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.

The method is injected with _add_delegate_accessors and the resulting signature on the object has *args and **kwargs instead of the actual date_format

[TomAugspurger]

I think it should be

formatted: ndarray
    NumPy array strings formatted according to `date_format`.

[jreback]

this is a duplicate of #20103 pls coordinate

[mircea-cosbuc]

Closed because of duplication