DOC: update the Period.start_time attribute docstring
State: closed

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.


Labels:
name: Docs
name: Period
description: Period data type

Comments:
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@@            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
```

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| [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: |

------

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Added end_time to the see also.

Thanks @hammadmashkoor .