DOC: update the pandas.Series.str.strip docstring

[techedlaksh]

Checklist for the pandas documentation sprint (ignore this if you are doing
an unrelated PR):

• PR title is "DOC: update the docstring"
• The validation script passes: scripts/validate_docstrings.py <your-function-or-method>
• The PEP8 style check passes: git diff upstream/master -u -- "*.py" | flake8 --diff
• The html version looks good: python doc/make.py --single <your-function-or-method>
• 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]

Codecov Report

Merging #20628 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            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)	⬆️
#single	41.9% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/core/strings.py	98.32% <ø> (ø)	⬆️
pandas/util/testing.py	84.73% <0%> (+0.2%)	⬆️

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[datapythonista]

Examples section goes after Returns

[datapythonista]

You can write sentences explaining what you're doing without making then Python comments. Check the docstring guide or other docstrings already improved.

[datapythonista]

I'd create a single Series with all the values you want to use in the examples, and then apply all operations to it.

[datapythonista]

Not necessarily whitespaces.

[datapythonista]

Seems like #20863 is also fixing the same docstring, and IMO looks better. I think we can close this one.

[mroeschke]

Thanks @techedlaksh. Unfortunately it appears that there was some overlap with #20863; sorry about that. Feel free to work on other docstrings, always appreciated!