Add cover note to consistency section

Make it clear that the new consistency rules should match the existing rules for any types not using the new features.

peps/pep-0705.rst

@@ -371,6 +371,8 @@ Finally, subclasses can have ``other_keys=Never`` even if the superclass does no
 Type consistency
 ----------------
 
+*This section updates the type consistency rules introduced in :pep:`589` to cover the new features in this PEP. In particular, any pair of types that do not use the new features will be consistent under these new rules if (and only if) they were already consistent.*
+
 A TypedDict type with ``other_keys=Never`` is consistent with ``Mapping[str, V]``, where ``V`` is the union of all its value types. For instance, the following type is consistent with ``Mapping[str, int | str]``::
 
     class Person(TypedDict, other_keys=Never):