DOC: Update the pandas.Series.str.count() docstring (Delhi)
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"
- [ ] 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.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.