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
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 

Doc/reference/datamodel.rst

@@ -154,11 +154,16 @@ NotImplemented
 
    This type has a single value.  There is a single object with this value. This
    object is accessed through the built-in name ``NotImplemented``. Numeric methods
-   and rich comparison methods may return this value if they do not implement the
+   and rich comparison methods should return this value if they do not implement the
    operation for the operands provided.  (The interpreter will then try the
    reflected operation, or some other fallback, depending on the operator.)  Its
    truth value is true.
 
+   See
+   :ref:`implementing-the-arithmetic-operations`
+   for more details.
+
+
 Ellipsis
    .. index:: object: Ellipsis