DOCS: docstrings formatting errors in pandas.PeriodIndex
[pandas/core/indexes/period.py](https://github.com/pandas-dev/pandas/blob/04893a954b91574279c402e8730a4b5fae2ae9e1/pandas/core/indexes/period.py)

#28602 
#27977

```
./scripts/validate_docstrings.py --errors=PR09
```
```
pandas.PeriodIndex: Parameter "data" description should finish with "."
pandas.PeriodIndex: Parameter "copy" description should finish with "."
pandas.PeriodIndex: Parameter "freq" description should finish with "."
pandas.PeriodIndex: Parameter "periods" description should finish with "."
pandas.PeriodIndex: Parameter "end" description should finish with "."
pandas.PeriodIndex: Parameter "tz" description should finish with "."
pandas.PeriodIndex.asfreq: Parameter "freq" description should finish with "."
pandas.PeriodIndex.to_timestamp: Parameter "freq" description should finish with "."
```

```
./scripts/validate_docstrings.py pandas.PeriodIndex
```
```
Summary should fit in a single line
Parameters {**fields, name, ordinal} not documented
Unknown parameters {month, quarter, second, day, year, minute, hour}
Parameter "data" description should finish with "."
Parameter "copy" description should finish with "."
Parameter "freq" description should finish with "."
Parameter "periods" description should finish with "."
Parameter "end" description should finish with "."
Parameter "year" has no description
Parameter "month" has no description
Parameter "quarter" has no description
Parameter "day" has no description
Parameter "hour" has no description
Parameter "minute" has no description
Parameter "second" has no description
Parameter "tz" description should finish with "."
Parameter "dtype" has no description
Examples do not pass tests
flake8 error: F821 undefined name 'year_arr' (2 times)
```
and a failed example on line 89

```
./scripts/validate_docstrings.py pandas.PeriodIndex.asfreq
```

```
Parameters {*args, **kwargs} not documented
Unknown parameters {freq, how}
Parameter "freq" description should start with a capital letter
Parameter "freq" description should finish with "."
Parameter "how" description should start with a capital letter
The first line of the Returns section should contain only the type, unless multiple values are being returned
Return value has no description
```

```
./scripts/validate_docstrings.py pandas.PeriodIndex.to_timestamp
```

```
Parameters {**kwargs, *args} not documented
Unknown parameters {freq, how}
Parameter "freq" description should finish with "."
Parameter "how" has no description
Return value has no description
```