DOC: update the Series.str.join 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.str.join) ######################
################################################################################

Join lists contained as elements in the Series/Index with passed delimiter.

If the elements of a Series are lists themselves, join the content of these
lists using the delimiter passed to the function.
This function is an equivalent to :meth:`str.join`.

Parameters
----------
sep : str
    Delimiter to use between list entries.

Returns
-------
Series/Index of objects

Notes
-----
If any of the lists does not contain string objects the result of the join
will be `NaN`.

See Also
--------
str.join : Standard library version of this method.
Series.str.split : Split strings around given separator/delimiter.

Examples
--------

Example with a list that contains non-string elements.

>>> s = pd.Series([['lion', 'elephant', 'zebra'],
...                [1.1, 2.2, 3.3],
...                ["cat", np.nan, "dog"],
...                ["cow", 4.5, "goat"]])
>>> s
0    [lion, elephant, zebra]
1            [1.1, 2.2, 3.3]
2            [cat, nan, dog]
3           [cow, 4.5, goat]
dtype: object

Join all lists using an '-', the list of floats will become a NaN.

>>> s.str.join('-')
0    lion-elephant-zebra
1                    NaN
2                    NaN
3                    NaN
dtype: object

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

Docstring for "pandas.Series.str.join" 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.


