#6540: Update documentation of default values in gc.mli

http://caml.inria.fr/mantis/view.php?id=6540

stdlib/gc.mli and byterun/config.h disagree on the default size of the minor heap

byterun/config.h:

```c
/* Default size of the minor zone. (words) */
#define Minor_heap_def 262144
```

doesn't match stdlib/gc.mli:

```ocaml
{ mutable minor_heap_size : int;
  (** The size (in words) of the minor heap. Changing
     this parameter will trigger a minor collection. Default: 32k. *)
```

ditto

```c
/* Default maximum size of the stack (words). */
#define Max_stack_def (1024 * 1024)
```

vs

```ocaml
mutable stack_limit : int;
(** The maximum size of the stack (in words). This is only
    relevant to the byte-code runtime, as the native code runtime
    uses the operating system's stack. Default: 256k. *)
```