bpo-41798: Allocate _socket module C API on the heap (GH-24126)

diff --git a/Modules/socketmodule.h b/Modules/socketmodule.h
index ba2c9f5..e4f375d 100644
--- a/Modules/socketmodule.h
+++ b/Modules/socketmodule.h
@@ -342,7 +342,8 @@ typedef struct {
 
 */
 
-/* C API for usage by other Python modules */
+/* C API for usage by other Python modules.
+ * Always add new things to the end for binary compatibility. */
 typedef struct {
     PyTypeObject *Sock_Type;
     PyObject *error;