Describe the default hash correctly, and mark a couple of CPython implementation details
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 9f0b112..0510d8e 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -35,12 +35,19 @@
Every object has an identity, a type and a value. An object's *identity* never
changes once it has been created; you may think of it as the object's address in
memory. The ':keyword:`is`' operator compares the identity of two objects; the
-:func:`id` function returns an integer representing its identity (currently
-implemented as its address). An object's :dfn:`type` is also unchangeable. [#]_
+:func:`id` function returns an integer representing its identity.
+
+.. impl-detail::
+
+ For CPython, ``id(x)`` is the memory address where ``x`` is stored.
+
An object's type determines the operations that the object supports (e.g., "does
it have a length?") and also defines the possible values for objects of that
type. The :func:`type` function returns an object's type (which is an object
-itself). The *value* of some objects can change. Objects whose value can
+itself). Like its identity, an object's :dfn:`type` is also unchangeable.
+[#]_
+
+The *value* of some objects can change. Objects whose value can
change are said to be *mutable*; objects whose value is unchangeable once they
are created are called *immutable*. (The value of an immutable container object
that contains a reference to a mutable object can change when the latter's value
@@ -1258,7 +1265,12 @@
User-defined classes have :meth:`__eq__` and :meth:`__hash__` methods
by default; with them, all objects compare unequal (except with themselves)
- and ``x.__hash__()`` returns ``id(x)``.
+ and ``x.__hash__()`` returns an appropriate value such that ``x == y``
+ implies both that ``x is y`` and ``hash(x) == hash(y)``.
+
+ .. impl-detail::
+
+ CPython uses ``hash(id(x))`` as the default hash for class instances.
Classes which inherit a :meth:`__hash__` method from a parent class but
change the meaning of :meth:`__eq__` such that the hash value returned is no