explain __base__ attribute in the stdtypes doc

Author: furkanonder

Doc/library/stdtypes.rst

@@ -5445,6 +5445,15 @@ types, where they are relevant.  Some of these are not reported by the
    The tuple of base classes of a class object.
 
 
+.. attribute:: class.__base__
+
+  ``__base__`` corresponds to the :c:member:`~PyTypeObject.tp_base` in a
+  type object.  At the C level, Python has a single inheritance model
+  that determines the memory layout of instances.  There is a chain
+  involving base classes that contribute to the instance layout.
+  ``__base__`` is the base class that is involved in that chain.
+
+
 .. attribute:: definition.__name__
 
    The name of the class, function, method, descriptor, or