DOC: add section Raises to PeriodIndex docstring
we raise a ValueError: "freq not specified and cannot be inferred" if both freq and dtype are not specified in `PeriodIndex`

```
>>> pd.PeriodIndex(["2023-01-01", "2023-02-01", "2023-03-01", "2023-04-01"], freq="M")
PeriodIndex(['2023-01', '2023-02', '2023-03', '2023-04'], dtype='period[M]')

>>> pd.PeriodIndex(["2023-01-01", "2023-02-01", "2023-03-01", "2023-04-01"], dtype ="period[M]")
PeriodIndex(['2023-01', '2023-02', '2023-03', '2023-04'], dtype='period[M]')

>>> pd.PeriodIndex(["2023-01-01", "2023-02-01", "2023-03-01", "2023-04-01"])
ValueError: freq not specified and cannot be inferred
```
I added a section "Raises" to `PeriodIndex` documentation.
Thanks @natmokval 