Docs: fix built-in function __self__ attribute descriptions

Doc/library/inspect.rst

@@ -319,6 +319,8 @@ attributes (see :ref:`import-mod-attrs` for module attributes):
 +-----------------+-------------------+---------------------------+
 |                 | __self__          | instance to which a       |
 |                 |                   | method is bound, or       |
+|                 |                   | module object a function  |
+|                 |                   | was defined in, or        |
 |                 |                   | ``None``                  |
 +-----------------+-------------------+---------------------------+
 

Doc/reference/datamodel.rst

@@ -863,7 +863,8 @@ determined by the C function. Special read-only attributes:
 * :attr:`!__doc__` is the function's documentation string, or ``None`` if
   unavailable. See :attr:`function.__doc__`.
 * :attr:`!__name__` is the function's name. See :attr:`function.__name__`.
-* :attr:`!__self__` is set to ``None`` (but see the next item).
+* :attr:`!__self__` is the module object the function was defined in or
+  ``None`` if unavailable.
 * :attr:`!__module__` is the name of
   the module the function was defined in or ``None`` if unavailable.
   See :attr:`function.__module__`.

Lib/inspect.py

@@ -441,7 +441,8 @@ def isbuiltin(object):
     Built-in functions and methods provide these attributes:
         __doc__         documentation string
         __name__        original name of this function or method
-        __self__        instance to which a method is bound, or None"""
+        __self__        instance to which a method is bound, or module object
+                        a function was defined in, or None"""
     return isinstance(object, types.BuiltinFunctionType)
 
 def ismethodwrapper(object):