DOC: Add freq strings reference link for Period
#### Location of the documentation

[pandas.Period](https://pandas.pydata.org/docs/reference/api/pandas.Period.html)

#### Documentation problem

The doc does not provide much information on the possible offset-aliases for the `freq` parameter

#### Suggested fix for documentation

A link to [offset-aliases](https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#offset-aliases) for the `freq` parameter for quick reference
