Doc: Update the DatetimeIndex.strftime docstring
Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

- [ ] PR title is "DOC: update the <your-function-or-method> 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.DatetimeIndex.strftime)  ##################
################################################################################

Convert to specified date_format.

Return an array of formatted strings 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
-------
numpy.ndarray
    n-dimensional array of formatted strings

See Also
--------
DatetimeIndex.normalize : Return DatetimeIndex with times to midnight.
DatetimeIndex.round : Round the DatetimeIndex to the specified freq.
DatetimeIndex.floor : Floor the DatetimeIndex to the specified freq.

Examples
--------
>>> data = ['2015-05-01 18:47:05.060000','2014-05-01 18:47:05.110000']
>>> df = pd.DataFrame(data, columns=['date'])
>>> df.date = pd.to_datetime(df.date)
>>> df.date[1]
Timestamp('2014-05-01 18:47:05.110000')
>>> df.date[1].strftime('%d-%m-%Y')
'01-05-2014'
>>> df.date[1].strftime('%B %d, %Y, %r')
'May 01, 2014, 06:47:05 PM'

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

Docstring for "pandas.DatetimeIndex.strftime" correct. :)
```

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.


Checklist for other PRs (remove this part if you are doing a PR for the pandas documentation sprint):