add return-description-missing violation

[jshwi]

$ cat example.py
# def normalize(ticker: str) -> str:
#     """Normalize ticket names.
#
#     :param ticket: name to normalize.
#     :return:
#     """

should raise:

$ docsig example.py
# example.py:2 in normalize
#     SIG506: description missing from return (return-description-missing)