add Changes entry and documentation comment

Changes

@@ -63,6 +63,11 @@ Working version
   due to memory resource exhaustion.  It was previous always a `Failure`.
   (Anil Madhavapeddy, review by David Allsopp)
 
+- #12318: GC: simplify the meaning of custom_minor_max_size: blocks with
+  out-of-heap memory above this limit are now allocated directly in
+  the major heap.
+  (Damien Doligez, report by Stephen Dolan, review by Gabriel Scherer)
+
 ### Code generation and optimizations:
 
 - #11239: on x86-64 and RISC-V, reduce alignment of OCaml stacks from 16 to 8.

runtime/caml/custom.h

@@ -57,6 +57,10 @@ CAMLextern value caml_alloc_custom(const struct custom_operations * ops,
                                    mlsize_t mem, /*resources consumed*/
                                    mlsize_t max  /*max resources*/);
 
+/* [caml_alloc_custom_mem] allocates a custom block with dependent memory
+   (memory outside the heap that will be reclaimed when the block is
+   finalized). If [mem] is greater than [custom_minor_max_size] (see gc.mli)
+   the block is allocated directly in the major heap. */
 CAMLextern value caml_alloc_custom_mem(const struct custom_operations * ops,
                                        uintnat size, /*size in bytes*/
                                        mlsize_t mem  /*memory consumed*/);