DOC: update the pandas.Series.dt.is_year_end docstring

[janelle]

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.is_year_end) ###################
################################################################################

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

Returns
-------
is_year_end : Series of boolean.

See Also
--------
is_year_start : Return a boolean indicating whether the date is the first 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_end
0    False
1     True
2    False
dtype: bool

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

Errors found:
	No extended summary 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.

• No extended summary required

[pep8speaks]

Hello @janelle! Thanks for updating the PR.

• In the file pandas/core/indexes/datetimes.py, following are the PEP8 issues :

Line 1793:80: E501 line too long (80 > 79 characters)

Comment last updated on March 14, 2018 at 18:55 Hours UTC

[TomAugspurger]

This should be a single line. Does

Return a boolean array of whether the date is the year's last day.

fit on a single line?

[jorisvandenbossche]

Boolean array is not correct in case of Series ...

(but I agree that just "boolean" does not reflect is returns a boolean for each element)

[TomAugspurger]

Perhaps just

Return an indicator for whether each date is the last day of the year.

[jorisvandenbossche]

Yes, that's better. Or maybe even shorter "Indicate whether ..." instead of "Return an indicator whether .." ?

[TomAugspurger]

Could you add an example with just a DatetimeIndex? Can be the same values, and don't have to show the values again,

>>> pd.date_range('2017-12-30", periods=3).is_year_end

and show the output

[TomAugspurger]

Ah, this return type depends on the type of self. So I think

is_year_end : Series or DatetimeIndex
The same type as the original data. Series will have the same
name and index. DatetimeIndex will have the same name.

[TomAugspurger]

See comment #20275 (comment) (once we figure out how to handle these)

[TomAugspurger]

Updated. Fixed some some See Alsos in the other docstrings where we said method instead of property.

Thanks again @janelle!