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 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
Hello @feiphoon! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on August 07, 2018 at 17:24 Hours UTC Looks great, minor problem, the line ([Line 1108:80](https://github.com/pandas-dev/pandas/blob/ea35874b9713964bbce6f494af2eb777d5386746/pandas/core/strings.py#L1108)) is too long so may be break it up in 2 lines  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=h1) Report
> Merging [#22174](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6e1e1e40f7e474886f46b88edfd4c5bc22a35b14?src=pr&el=desc) will **increase** coverage by `0.09%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22174/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22174      +/-   ##
==========================================
+ Coverage   92.06%   92.15%   +0.09%     
==========================================
  Files         169      169              
  Lines       50694    51312     +618     
==========================================
+ Hits        46671    47288     +617     
- Misses       4023     4024       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.58% <ø> (+0.11%)` | :arrow_up: |
| #single | `42.37% <ø> (+0.05%)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.63% <ø> (ø)` | :arrow_up: |
| [pandas/tseries/offsets.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvb2Zmc2V0cy5weQ==) | `97.15% <0%> (ø)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `70.65% <0%> (ø)` | :arrow_up: |
| [pandas/core/internals/blocks.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzL2Jsb2Nrcy5weQ==) | `94.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `91.7% <0%> (+0.04%)` | :arrow_up: |
| [pandas/core/indexes/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9iYXNlLnB5) | `96.43% <0%> (+0.05%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `85.9% <0%> (+0.2%)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.51% <0%> (+0.24%)` | :arrow_up: |
| [pandas/core/arrays/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3BlcmlvZC5weQ==) | `91.98% <0%> (+0.76%)` | :arrow_up: |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/22174/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=footer). Last update [6e1e1e4...e03704d](https://codecov.io/gh/pandas-dev/pandas/pull/22174?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Tagging @sebryan as pairing partner @datapythonista thank you for your help!

The Travis CI build failed on one job - ci/lint.sh 'Not Linting'. Is there a way to fix or rerun this? thanks @feiphoon  will merge on green Thank you @jreback @datapythonista !