DOC: Fix validation error RT01 in pandas/core/dtypes (pandas-dev#25356)

Author: ihsansecer

pandas/core/dtypes/dtypes.py

@@ -27,6 +27,11 @@ def register_extension_dtype(cls):
     This enables operations like ``.astype(name)`` for the name
     of the ExtensionDtype.
 
+    Returns
+    -------
+    callable
+        A class decorator.
+
     Examples
     --------
     >>> from pandas.api.extensions import register_extension_dtype

pandas/core/dtypes/inference.py

@@ -436,6 +436,10 @@ def is_hashable(obj):
     Distinguish between these and other types by trying the call to hash() and
     seeing if they raise TypeError.
 
+    Returns
+    -------
+    bool
+
     Examples
     --------
     >>> a = ([],)