DOC: Simplify docstrings by describing only the type returned

Also, fixes confusing `month_index` return name in `day_name()` method.

Author: Peque

pandas/core/arrays/datetimes.py

@@ -800,7 +800,7 @@ def month_name(self, locale=None):
 
         Returns
         -------
-        month_names : Index
+        Index
             Index of month names
 
         .. versionadded:: 0.23.0
@@ -826,7 +826,7 @@ def day_name(self, locale=None):
 
         Returns
         -------
-        month_names : Index
+        Index
             Index of day names
 
         .. versionadded:: 0.23.0