DOC: add back Timestamp unit kw to doc-string

Author: jreback

pandas/tslib.pyx

@@ -233,6 +233,8 @@ class Timestamp(_Timestamp):
         Offset which Timestamp will have
     tz : string, pytz.timezone, dateutil.tz.tzfile or None
         Time zone for time which Timestamp will have.
+    unit : string
+        numpy unit used for conversion, if ts_input is int or float
     """
 
     # Do not add ``dayfirst`` and ``yearfist`` to Timestamp based on the discussion