DOCS: Errors PR08 PR09 in pandas.Period
Subset of #28602 (PR09) and #25232 (PR08) for #27977 (docstrings validation). 

pandas/_libs/tslibs/period.pyx

```
python scripts/validate_docstrings.py --errors=PR08
python scripts/validate_docstrings.py --errors=PR09
python scripts/validate_docstrings.py pandas.Period

```

```
pandas.Period: Parameter "value" description should finish with "."
pandas.Period: Parameter "freq" description should finish with "."
pandas.Period.asfreq: Parameter "how" description should finish with "."
pandas.Period.to_timestamp: Parameter "freq" description should finish with "."
pandas.Period.to_timestamp: Parameter "how" description should finish with "."
pandas.PeriodDtype: Parameter "freq" description should finish with "."
pandas.Period.to_timestamp: Parameter "how" description should start with a capital letter
```

```
10 Errors found:
	Parameters {ordinal} not documented
	Parameter "value" description should finish with "."
	Parameter "freq" 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
```