chore: link to C API howto for Python 3.8 quirk

src/zope/interface/_zope_interface_coptimizations.c

@@ -238,7 +238,10 @@ typedef struct
 static int
 SB_traverse(SB* self, visitproc visit, void* arg)
 {
-/* Visit our 'tp_type' only on Python >= 3.9 */
+/* Visit our 'tp_type' only on Python >= 3.9, per
+ * https://docs.python.org/3/howto/isolating-extensions.html
+ * #tp-traverse-in-python-3-8-and-lower
+ */
 #if PY_VERSION_HEX > 0x03090000
     Py_VISIT(Py_TYPE(self));
 #endif
@@ -1091,6 +1094,10 @@ typedef struct
 static int
 LB_traverse(LB* self, visitproc visit, void* arg)
 {
+/* Visit our 'tp_type' only on Python >= 3.9, per
+ * https://docs.python.org/3/howto/isolating-extensions.html
+ * #tp-traverse-in-python-3-8-and-lower
+ */
 #if PY_VERSION_HEX > 0x03090000
     Py_VISIT(Py_TYPE(self));
 #endif