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

Test if the start of each string element matches a pattern.

Return a Series of booleans indicating whether the given pattern matches
the start of each string element.
Equivalent to :meth: `str.startswith`.

Parameters
----------
pat : string
    Character sequence.
na : object, default NaN
    Character sequence shown if element tested is not a string.

Returns
-------
startswith : Series/array of boolean values

Examples
--------
>>> s = pd.Series(['bat', 'bear', 'cat'])
>>> s.str.startswith('b')
0     True
1     True
2    False
dtype: bool
>>> s = pd.Series(['bat', 'bear', 'cat', np.nan])
>>> s.str.startswith('b', na='not_a_string')
0            True
1            True
2           False
3    not_a_string
dtype: object

See Also
--------
endswith : same as startswith, but tests the end of string

################################################################################
################################## Validation ##################################
################################################################################
```

If the validation script still gives errors, but you think there is a good reason
to deviate in this case (and there are certainly such cases), please state this
explicitly.
