Update period.py

proposed fix for #4699: Period() docstring inconsistent with code when freq has a mult != 1
DOC: added "Must be a singlular rule-code (e.g. 5T is not allowed)."
Merge pull request #5057 from JackKelly/master

DOC: proposed fix for #4699: Period() docstring inconsistent with code when freq has a mult != 1