DOC: update the pandas.Series.str.strip 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.strip)  #####################
################################################################################

Strip whitespaces from string in Series.

Strip whitespace (including newlines) or given string from
each string in the Series/Index from left and right sides. Equivalent to
:meth:`str.strip`.

Parameters
----------
to_strip : str or unicode
    String or unicode to strip in the given string.

Examples
--------
>>> # strip method
>>> s = pd.Series(['   This is a Test 1  '])
>>> s
0    This is a Test 1
dtype: object
>>> s = s.str.strip()
>>> s
0    This is a Test 1
dtype: object
>>> # lstrip method
>>> s1 = pd.Series(['This is another Test'])
>>> s1
0    This is another Test
dtype: object
>>> s1 = s1.str.lstrip('This')
>>> s1
0     is another Test
dtype: object
>>> # rstrip method
>>> s2 = pd.Series(['This is the last test'])
>>> s2
0    This is the last test
dtype: object
>>> s2 = s2.str.rstrip('test')
>>> s2
0     This is the last
dtype: object

Returns
-------
stripped : Series/Index of objects

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

Errors found:
	See Also section not found


```
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20628?src=pr&el=h1) Report
> Merging [#20628](https://codecov.io/gh/pandas-dev/pandas/pull/20628?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6d610a4d9393c0d0335267dc3252ccabb9e51e43?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20628/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/20628?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20628      +/-   ##
==========================================
+ Coverage   91.82%   91.82%   +<.01%     
==========================================
  Files         153      153              
  Lines       49256    49256              
==========================================
+ Hits        45227    45229       +2     
+ Misses       4029     4027       -2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.21% <ø> (ø)` | :arrow_up: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20628?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20628/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20628/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.2%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20628?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/20628?src=pr&el=footer). Last update [6d610a4...f6cc70f](https://codecov.io/gh/pandas-dev/pandas/pull/20628?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Seems like #20863 is also fixing the same docstring, and IMO looks better. I think we can close this one. Thanks @techedlaksh. Unfortunately it appears that there was some overlap with #20863; sorry about that. Feel free to work on other docstrings, always appreciated!