Add ref to docstring of Series:dtype (pandas-dev#24421)

Author: thoo

pandas/core/series.py

@@ -131,6 +131,7 @@ class Series(base.IndexOpsMixin, generic.NDFrame):
     dtype : str, numpy.dtype, or ExtensionDtype, optional
         dtype for the output Series. If not specified, this will be
         inferred from `data`.
+        See the :ref:`user guide <basics.dtypes>` for more usages.
     copy : bool, default False
         Copy input data.
     """