Explain __base__ attribute in the docs

Author: furkanonder

Doc/c-api/object.rst

@@ -245,6 +245,46 @@ Object Protocol
    class, are considered classes.  However, objects can override this by having
    a :attr:`__bases__` attribute (which must be a tuple of base classes).
 
+   Another function, :attr:`__base__` that is specific to Cpython and also
+   exists in Jython and PyPy can also be used on a class inheriting from one
+   or more classes. When such a class takes arguments in the correct order,
+   then starting leftmost.
+
+   Let's look at the example cases;
+
+   >>> class A(object): pass
+   ...
+   >>> class B(A): pass
+   ...
+   >>> class C(int): pass
+   ...
+
+   The first user-defined class that either inherits from the instance
+   of a built-in type other than object or inherits from another user
+   defined class (single or multiple inheritance) that does so or in the
+   absence of the above class.
+
+   >>> class D(B, A, C): pass
+   ...
+   >>> D.__base__
+   <class '__main__.C'>
+   >>>
+
+   A built-in type that is not an object or in the absence of the above class.
+
+   >>> class D(B, A, int): pass
+   ...
+   >>> D.__base__
+   <type 'int'>
+
+   The first user defined class that inherits  either an object or
+   derives from a class (directly or indirectly) that inherits an
+   object is the value returned by the :attr:`__base__` function.
+
+   >>> class D(B, A): pass
+   ...
+   >>> D.__base__
+   <class '__main__.B'>
 
 .. c:function:: int PyObject_IsInstance(PyObject *inst, PyObject *cls)