DOC: update the pandas.Series.dt.is_leap_year 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_leap_year)  ##################
################################################################################

Return a boolean indicating if the date belongs to a leap year.

A leap year is a year, occurring every four years, which has 366 days
(instead of 365) including 29th of February as an intercalary day.

Returns
-------
is_leap_year : Series of boolean

Examples
--------
>>> import pandas as pd
>>> dates = pd.date_range("2012-01-01", "2015-01-01", freq="Y")
>>> dates_series = pd.Series(dates)
>>> dates_series
0   2012-12-31
1   2013-12-31
2   2014-12-31
dtype: datetime64[ns]
>>> dates_series.dt.is_leap_year
0     True
1    False
2    False
dtype: bool

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

Errors found:
	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.

See Also is not relevant for this function as there is no other leap year related functions.
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```diff
@@            Coverage Diff             @@
##           master   #20150      +/-   ##
==========================================
- Coverage    91.8%   91.72%   -0.08%     
==========================================
  Files         152      150       -2     
  Lines       49206    49152      -54     
==========================================
- Hits        45172    45086      -86     
- Misses       4034     4066      +32
```

| Flag | Coverage Δ | |
|---|---|---|
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| #single | `41.84% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20150?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
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| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/20150/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.03% <0%> (-0.2%)` | :arrow_down: |
| [pandas/core/reshape/melt.py](https://codecov.io/gh/pandas-dev/pandas/pull/20150/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9tZWx0LnB5) | `97.19% <0%> (-0.15%)` | :arrow_down: |
| ... and [28 more](https://codecov.io/gh/pandas-dev/pandas/pull/20150/diff?src=pr&el=tree-more) | |

------

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