#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:

    /* Default size of the minor zone. (words) */
    #define Minor_heap_def 262144

doesn't match stdlib/gc.mli:

    { mutable minor_heap_size : int;
      (** The size (in words) of the minor heap. Changing
         this parameter will trigger a minor collection. Default: 32k. *)

ditto

    /* Default maximum size of the stack (words). */
    #define Max_stack_def (1024 * 1024)

vs

    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. *)