DOC: update the Period.start_time attribute 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>`
- [ ] It has been proofread on language by another sprint participant

Please include the output of the validation script below between the "```" ticks:

```
(pandas_dev) root@kalih:~/pythonpanda/pandas# python scripts/validate_docstrings.py pandas.Period.start_time

################################################################################
##################### Docstring (pandas.Period.start_time) #####################
################################################################################

Get the Timestamp for the start of the period.

Returns
-------
Timestamp

See also
--------
Period.dayofyear : Return the day of year.
Period.daysinmonth : Return the days in that month.
Period.dayofweek : Return the day of the week.

Examples
--------
>>> period = pd.Period('2012-1-1', freq='D')
>>> period
Period('2012-01-01', 'D')

>>> period.start_time
Timestamp('2012-01-01 00:00:00')
>>> period.end_time
Timestamp('2012-01-01 23:59:59.999999999')

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

Errors found:
	No extended summary found


```

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/20325?src=pr&el=h1) Report
> Merging [#20325](https://codecov.io/gh/pandas-dev/pandas/pull/20325?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/3783ccc694ef93c0cf159ca1ad8e69a67d828bf0?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20325/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20325?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #20325      +/-   ##
==========================================
- Coverage   91.76%   91.73%   -0.03%     
==========================================
  Files         150      150              
  Lines       49151    49151              
==========================================
- Hits        45102    45090      -12     
- Misses       4049     4061      +12
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.12% <ø> (-0.03%)` | :arrow_down: |
| #single | `41.9% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20325?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/20325/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.07% <0%> (-1.74%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20325?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/20325?src=pr&el=footer). Last update [3783ccc...5ba42a7](https://codecov.io/gh/pandas-dev/pandas/pull/20325?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Added end_time to the see also.

Thanks @hammadmashkoor .