DOC: added "Must be a singlular rule-code (e.g. 5T is not allowed)."

Author: JackKelly

pandas/tseries/period.py

@@ -51,7 +51,7 @@ class Period(PandasObject):
     value : Period or compat.string_types, default None
         The time period represented (e.g., '4Q2005')
     freq : str, default None
-        e.g., 'B' for businessday
+        e.g., 'B' for businessday. Must be a singlular rule-code (e.g. 5T is not allowed).
     year : int, default None
     month : int, default 1
     quarter : int, default None