DOC: update the Index.isin 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.Index.isin)  ########################
################################################################################

Boolean array on existence of index values in `values`.

Compute boolean array of whether each index value is found in the
passed set of values. Length of the returned boolean array matches
the length of the index.

Parameters
----------
values : set or list-like
    Sought values.

    .. versionadded:: 0.18.1

    Support for values as a set.

level : str or int, optional in the case of Index, compulsory on
    MultiIndex
    Name or position of the index level to use (if the index is a
    MultiIndex).

Returns
-------
is_contained : ndarray (boolean dtype)

See also
--------
DatetimeIndex.isin : an Index of :class:`Datetime` s
TimedeltaIndex : an Index of :class:`Timedelta` s
PeriodIndex : an Index of :class:`Period` s
MultiIndex.isin : Same for `MultiIndex`
NumericIndex.isin : Same for `Int64Index`, `UInt64Index`,
                    `Float64Index`

Notes
-----
If `level` is specified:

- if it is the name of one *and only one* index level, use that level;
- otherwise it should be a number indicating level position.

Examples
--------
>>> idx = pd.Index([1,2,3])
>>> idx
Int64Index([1, 2, 3], dtype='int64')

Check whether a value is in the Index:
>>> idx.isin([1])
array([ True, False, False])

>>> midx = pd.MultiIndex.from_arrays([[1,2,3],
...                                    ['red','blue','green']],
...                                    names=('number', 'color'))
>>> midx
MultiIndex(levels=[[1, 2, 3], ['blue', 'green', 'red']], labels=[[0, 1, 2], [2, 0, 1]], names=['number', 'color'])

Check whether a string index value is in the 'color' level of the
MultiIndex:

>>> midx.isin(['red'],'color')
array([ True, False, False])

>>> dates = ['3/11/2000', '3/12/2000', '3/13/2000']
>>> dti = pd.to_datetime(dates)
>>> dti
DatetimeIndex(['2000-03-11', '2000-03-12', '2000-03-13'], dtype='datetime64[ns]', freq=None)

Check whether a datetime index value is in the DatetimeIndex:

>>> dti.isin(['3/11/2000'])
array([ True, False, False])

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

Docstring for "pandas.Index.isin" 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.
