Update PEP 620 (#1450)

pep-0620.rst

@@ -50,19 +50,17 @@ released in 2009, has been removed in the Python 3.8 development cycle.
 But the change broke too many C extensions and had to be reverted before
 3.8 final release. Finally, the member was removed again in Python 3.9.
 
-C extensions rely on the ability to access directly structure members,
+C extensions rely on the ability to access structure members,
 indirectly through the C API, or even directly. Modifying structures
 like ``PyListObject`` cannot be even considered.
 
 The ``PyTypeObject`` structure is the one which evolved the most, simply
 because there was no other way to evolve CPython than modifying it.
 
-In the C API, all Python objects are passed as ``PyObject*``: a pointer
-to a ``PyObject`` structure. Experimenting tagged pointers in CPython is
-blocked by the fact that a C extension can technically dereference a
-``PyObject*`` pointer and access ``PyObject`` members. Small "objects"
-can be stored as a tagged pointer with no concrete ``PyObject``
-structure.
+A C extension can technically dereference a ``PyObject*`` pointer and
+access ``PyObject`` members. This prevents experiments like tagged
+pointers (storing small values as ``PyObject*`` which does not point to
+a valid ``PyObject`` structure).
 
 Replacing Python garbage collector with a tracing garbage collector
 would also need to remove ``PyObject.ob_refcnt`` reference counter,
@@ -345,9 +343,18 @@ Macros converted to regular functions in Python 3.9:
 Make structures opaque
 ----------------------
 
-All structures of the C API should become opaque: C extensions must
-use getter or setter functions to get or set structure members. For
-example, ``tuple->ob_item[0]`` must be replaced with
+The following structures of the C API become opaque:
+
+* ``PyInterpreterState``
+* ``PyThreadState``
+* ``PyGC_Head``
+* ``PyTypeObject``
+* ``PyObject`` and ``PyVarObject``
+* ``PyTypeObject``
+* All types which inherit from ``PyObject`` or ``PyVarObject``
+
+C extensions must use getter or setter functions to get or set structure
+members. For example, ``tuple->ob_item[0]`` must be replaced with
 ``PyTuple_GET_ITEM(tuple, 0)``.
 
 To be able to move away from reference counting, ``PyObject`` must