DOC: update the pandas.Series.dt.is_year_start 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.is_year_start) ##################
################################################################################

Return a boolean indicating whether the date is the first day of the
year.

Returns
-------
is_year_start : Series of boolean.

See Also
--------
is_year_end : Return a boolean indicating whether the date is the
    last day of the year.

Examples
--------
>>> dates = pd.Series(pd.date_range("2017-12-30", periods=3))
>>> dates
0   2017-12-30
1   2017-12-31
2   2018-01-01
dtype: datetime64[ns]
>>> dates.dt.is_year_start
0    False
1    False
2    True
dtype: bool

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

Errors found:
    No summary found (a short summary in a single line should be present at the beginning of the docstring)

```

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.

* short summary exceeds one line by one word, hope that is ok
