DOC: Update the pandas.Series.str.count() docstring (Delhi)

[Ridhwanluthra]

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.count)  #####################
################################################################################

Count occurrences of pattern in each string of the Series/Index.

This function is used to count the number of times a particular regex
pattern is repeated in each of the string elements of the
:class:`~pandas.Series`.

Parameters
----------
pat : str
    Valid regular expression.
flags : int, default 0, meaning no flags
    Flags for re module, e.g. re.IGNORECASE.

Returns
-------
counts : Series/Index of integer values

Notes
-----
Some characters need to be escaped when passing in pat.
eg. '$' has a special meaning in regex and must be escaped when finding
specifically this char.

Examples
--------
Take a look at
`this link <https://docs.python.org/3/howto/regex.html#compilation-flags>`_
for the list of all possible flags that can be used.

>>> s = pd.Series(['A', 'B', 'Aaba', 'Baca', np.nan, 'CABA', 'cat'])
>>> s.str.count('a')
0    0.0
1    0.0
2    2.0
3    2.0
4    NaN
5    0.0
6    1.0
dtype: float64

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

Errors found:
	Errors in parameters section
		Parameters {'kwargs'} not documented
	See Also section not found

As discussed in the gitter channel it was suggested to ignore kwargs for now.
Not sure what should be a part of see also here.

[TomAugspurger]

Maybe move this link to be under the flags parameter?

[Ridhwanluthra]

I like that idea, will do

[TomAugspurger]

Add an examle with special characters like $?

[Ridhwanluthra]

should I remove the note in the doc?

[codecov]

Codecov Report

Merging #20154 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #20154      +/-   ##
==========================================
- Coverage   91.72%   91.72%   -0.01%     
==========================================
  Files         150      150              
  Lines       49156    49152       -4     
==========================================
- Hits        45090    45086       -4     
  Misses       4066     4066

Flag	Coverage Δ
#multiple	90.11% <ø> (-0.01%)	⬇️
#single	41.84% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/strings.py	98.32% <ø> (ø)	⬆️
pandas/core/base.py	96.78% <0%> (-0.02%)	⬇️
pandas/core/series.py	93.85% <0%> (-0.01%)	⬇️
pandas/core/indexes/base.py	96.66% <0%> (-0.01%)	⬇️
pandas/core/indexes/multi.py	95.06% <0%> (ø)	⬆️
pandas/core/generic.py	95.84% <0%> (ø)	⬆️
pandas/plotting/_core.py	82.23% <0%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 840d432...93d2257. Read the comment docs.

[Ridhwanluthra]

@TomAugspurger please take a look at the update

[TomAugspurger]

Thanks!