improve period constructor docs (#47321)
improve period constructor docs (#47321)

* improve period docs

* Update pandas/_libs/tslibs/period.pyx

Co-authored-by: Matthew Roeschke <a890f767f8f4088da51818dce97c2cb936b19031@gmail.com>

Co-authored-by: Matthew Roeschke <a890f767f8f4088da51818dce97c2cb936b19031@gmail.com>
datapythonista/pandas,pandas-dev/pandas,datapythonista/pandas,datapythonista/pandas,datapythonista/pandas,pandas-dev/pandas,pandas-dev/pandas,pandas-dev/pandas