Add note to the __base__ attribute explanation

Author: furkanonder

Doc/c-api/object.rst

@@ -286,6 +286,8 @@ Object Protocol
    >>> D.__base__
    <class '__main__.B'>
 
+   Note that behavior of the ``__base__`` attribute is dependent on the CPython implementation.
+
 .. c:function:: int PyObject_IsInstance(PyObject *inst, PyObject *cls)
 
    Return ``1`` if *inst* is an instance of the class *cls* or a subclass of

Doc/library/stdtypes.rst

@@ -5453,6 +5453,8 @@ types, where they are relevant.  Some of these are not reported by the
   involving base classes that contribute to the instance layout.
   ``__base__`` is the base class that is involved in that chain.
 
+  .. note::
+    The behavior of the ``__base__`` attribute is dependent on the CPython implementation.
 
 .. attribute:: definition.__name__
 

Doc/reference/datamodel.rst

@@ -860,6 +860,9 @@ Custom classes
           involving base classes that contribute to the instance layout.
           ``__base__`` is the base class that is involved in that chain.
 
+          .. note::
+            The behavior of the ``__base__`` attribute is dependent on the CPython implementation.
+
       :attr:`__doc__`
          The class's documentation string, or ``None`` if undefined.