Issue22780: reword NotImplemented docs to emphasise should

Doc/library/constants.rst

@@ -26,9 +26,23 @@ A small number of constants live in the built-in namespace.  They are:
 
 .. data:: NotImplemented
 
-   Special value which should be returned by the special methods
-   (:meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, etc.) to indicate
-   that the operation is not implemented with respect to the other type.
+   Special value which should be returned by the binary special methods
+   (e.g. :meth:`__eq__`, :meth:`__lt__`, :meth:`__add__`, :meth:`__rsub__`,
+   etc.) to indicate that the operation is not implemented with respect to
+   the other type; may be returned by the in-place binary special methods
+   (e.g. :meth:`__imul__`, :meth:`__iand__`, etc.) for the same purpose.
+   Its truth value is true.
+
+.. note::
+
+   When ``NotImplemented`` is returned, the interpreter will then try the
+   reflected operation on the other type, or some other fallback, depending
+   on the operator.  If all attempted operations return ``NotImplemented``, the
+   interpreter will raise an appropriate exception.
+
+   See
+   :ref:`implementing-the-arithmetic-operations`
+   for more details.
 
 
 .. data:: Ellipsis

Doc/library/numbers.rst

@@ -110,6 +110,8 @@ those. You can add ``MyFoo`` between :class:`Complex` and
     MyFoo.register(Real)
 
 
+.. _implementing-the-arithmetic-operations:
+
 Implementing the arithmetic operations
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~