DOC: update the pandas.Series.dt.dayofweek 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:

```
################################################################################
#################### Docstring (pandas.Series.dt.dayofweek) ####################
################################################################################

The day of the week with Monday=0, Sunday=6.

Return the day of the week. It is assumed the week starts on
Monday, which is denoted by 0 and ends on Sunday which is denoted
by 6.

See Also
--------
pandas.Series.dt.dayofweek : Identical method.
pandas.Series.dt.weekday : Identical method.

Returns
-------
ndarray of integers indicating the day of the week

Examples
--------
>>> dates = pd.date_range("2016-12-31", "2017-01-08", freq="D")
>>> s = pd.Series(dates)
>>> day_name = s.dt.strftime("%A")
>>> pd.DataFrame({'dt': s, 'num': s.dt.weekday, 'day': day_name})
          dt  num        day
0 2016-12-31    5   Saturday
1 2017-01-01    6     Sunday
2 2017-01-02    0     Monday
3 2017-01-03    1    Tuesday
4 2017-01-04    2  Wednesday
5 2017-01-05    3   Thursday
6 2017-01-06    4     Friday
7 2017-01-07    5   Saturday
8 2017-01-08    6     Sunday

Note that `series.dt.dayofweek` and `series.dt.weekday` are the same.

>>> s.dt.dayofweek
0    5
1    6
2    0
3    1
4    2
5    3
6    4
7    5
8    6
dtype: int64
>>> s.dt.weekday
0    5
1    6
2    0
3    1
4    2
5    3
6    4
7    5
8    6
dtype: int64

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

Docstring for "pandas.Series.dt.dayofweek" correct. :)
```
Closing in favor of #21823 (the original fork/branch don't exist and can't fix the conflict in this PR).

Thanks @koaning for the contribution, and sorry for the delay.