Explain how old modules cannot load into a new interpreter.

pep-0353.txt

@@ -80,6 +80,10 @@ At places where a conversion from size_t/Py_ssize_t to
 int is necessary, the strategy for conversion is chosen
 on a case-by-case basis (see next section).
 
+To prevent loading extension modules that assume a 32-bit
+size type into an interpreter that has a 64-bit size type,
+Py_InitModule4 is renamed to Py_InitModule4_64.
+
 
 Conversion guidelines
 =====================