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.



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> Merging [#20463](https://codecov.io/gh/pandas-dev/pandas/pull/20463?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/02477dab1e4dfb26597806899f8ebf2ca019c30a?src=pr&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `n/a`.

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```diff
@@            Coverage Diff             @@
##           master   #20463      +/-   ##
==========================================
+ Coverage    91.8%   91.85%   +0.04%     
==========================================
  Files         152      152              
  Lines       49223    49231       +8     
==========================================
+ Hits        45191    45220      +29     
+ Misses       4032     4011      -21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (+0.04%)` | :arrow_up: |
| #single | `41.83% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20463?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
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| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.5% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.13% <0%> (+0.08%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.45% <0%> (+0.12%)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.55% <0%> (+0.41%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.61%)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `70.04% <0%> (+1.26%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/20463/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20463?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20463?src=pr&el=footer). Last update [02477da...323d072](https://codecov.io/gh/pandas-dev/pandas/pull/20463?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks for the feedback @datapythonista have updated the PR good points, thanks a lot 👍  Thanks @fdroessler !