Mark new macros change as potentially breaking

If an OCaml C library already defines some of the new `Val_none`,
`Some_val`, `Is_none`, `Is_some`, `caml_alloc_some`, or `Tag_some`
macros; then the C compiler will likely warn for macro redefinition,
even if the macro definition are identical.

This problem happens in [ocaml-mccs][1].

The proposed [fix][2] for libraries is to only define the macros when
compiling with OCaml strictly older than 4.12, i.e.:

    #if OCAML_VERSION < 41200
    #define ...
    #endif

[1]: ocaml-opam/ocaml-mccs#30
[2]: ocaml#5154 (comment)

Changes

@@ -38,8 +38,10 @@ Working version
   (KC Sivaramakrishnan, review by Stephen Dolan, Gabriel Scherer,
    and Xavier Leroy)
 
-- #9569: Add `Val_none`, `Some_val`, `Is_none`, `Is_some`, `caml_alloc_some`,
-  and `Tag_some`.
+* #5154, #9569, #9734: Add `Val_none`, `Some_val`, `Is_none`, `Is_some`,
+  `caml_alloc_some`, and `Tag_some`. As these macros are sometimes defined by
+  authors of C bindings, this change may cause warnings/errors in case of
+  redefinition.
   (Nicolás Ojeda Bär, review by Stephen Dolan, Gabriel Scherer, Mark Shinwell,
   and Xavier Leroy)