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.

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20458?src=pr&el=h1) Report
> Merging [#20458](https://codecov.io/gh/pandas-dev/pandas/pull/20458?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/02477dab1e4dfb26597806899f8ebf2ca019c30a?src=pr&el=desc) will **increase** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20458/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/20458?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20458      +/-   ##
==========================================
+ Coverage    91.8%   91.85%   +0.04%     
==========================================
  Files         152      152              
  Lines       49223    49231       +8     
==========================================
+ Hits        45191    45220      +29     
+ Misses       4032     4011      -21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <ø> (+0.04%)` | :arrow_up: |
| #single | `41.83% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20458?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/strings.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc3RyaW5ncy5weQ==) | `98.32% <ø> (ø)` | :arrow_up: |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | `96.2% <0%> (-0.02%)` | :arrow_down: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.85% <0%> (ø)` | :arrow_up: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.18% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.5% <0%> (ø)` | :arrow_up: |
| [pandas/io/formats/csvs.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvY3N2cy5weQ==) | `98.13% <0%> (+0.08%)` | :arrow_up: |
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.45% <0%> (+0.12%)` | :arrow_up: |
| [pandas/core/groupby.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ3JvdXBieS5weQ==) | `92.55% <0%> (+0.41%)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.73% <0%> (+0.61%)` | :arrow_up: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `70.04% <0%> (+1.26%)` | :arrow_up: |
| ... and [2 more](https://codecov.io/gh/pandas-dev/pandas/pull/20458/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20458?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/20458?src=pr&el=footer). Last update [02477da...9a6d018](https://codecov.io/gh/pandas-dev/pandas/pull/20458?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @TomAugspurger @datapythonista 
incorporated the feedback, I left a comment on the one I couldn't work out @datapythonista changed as per suggestions:) @datapythonista ok please check Thanks! Cool. Thanks for all the feedback!