Update Gc.control documentation to OCaml 5

[jmid]

While continuing on a Gc module test I spotted a bit more documentation needing updating on OCaml 5:

• 3 Gc.control fields (major_heap_increment, max_overhead, and window_size) are out of date, as they are unavailable. I've slightly adjusted the existing documentation of allocation_policy to make the four of them uniform
• stack_limit's documented default should reflect OCaml 5's much higher default value

I can see that the stack_limit was adjusted in #11238 as part of the 5.0 release
and that the manual was updated in #13066 as part of the 5.2 release.

For easy reference, the relevant parts underlying Gc.get does not extract fields 1, 4, 6, and 7:

ocaml/runtime/gc_ctrl.c

Lines 122 to 136 in f2ff4a4

	CAMLprim value caml_gc_get(value v)
	{
	CAMLparam0 (); /* v is ignored */
	CAMLlocal1 (res);
	res = caml_alloc_tuple (11);
	Store_field (res, 0, Val_long (Caml_state->minor_heap_wsz)); /* s */
	Store_field (res, 2, Val_long (caml_percent_free)); /* o */
	Store_field (res, 3, Val_long (atomic_load_relaxed(&caml_verb_gc))); /* v */
	Store_field (res, 5, Val_long (caml_max_stack_wsize)); /* l */
	Store_field (res, 8, Val_long (caml_custom_major_ratio)); /* M */
	Store_field (res, 9, Val_long (caml_custom_minor_ratio)); /* m */
	Store_field (res, 10, Val_long (caml_custom_minor_max_bsz)); /* n */
	CAMLreturn (res);
	}

and the same is true for Gc.set:

ocaml/runtime/gc_ctrl.c

Lines 155 to 163 in f2ff4a4

	CAMLprim value caml_gc_set(value v)
	{
	uintnat newminwsz = caml_norm_minor_heap_size (Long_val (Field (v, 0)));
	uintnat newpf = norm_pfree (Long_val (Field (v, 2)));
	uintnat new_verb_gc = Long_val (Field (v, 3));
	uintnat new_max_stack_size = Long_val (Field (v, 5));
	uintnat new_custom_maj = norm_custom_maj (Long_val (Field (v, 8)));
	uintnat new_custom_min = norm_custom_min (Long_val (Field (v, 9)));
	uintnat new_custom_sz = Long_val (Field (v, 10));

The default stack size is documented in these lines (note: it is 128M words):

ocaml/runtime/caml/config.h

Lines 195 to 197 in f2ff4a4

	/* Default maximum size of the stack (words). */
	/* (1 Gib for 64-bit platforms, 512 Mib for 32-bit platforms) */
	#define Max_stack_def (128 * 1024 * 1024)

[jmid]

Rebased on trunk to avoid merge conflicts in Changes

[gasche]

I warmly welcome documentation improvements. Thanks! Approved.

[jmid]

Thanks, I've updated Changes accordingly.

Would be good to cherry-pick this one to 5.3 along with #13424 (the latter first then this one, to avoid Changes conflicts)