Document non-trivial function Code.constant_equal

Co-authored-by: Jérome Vouillon <jerome.vouillon@gmail.com>

Author: OlivierNicole

compiler/lib/code.mli

@@ -185,6 +185,9 @@ type constant =
   | Tuple of int * constant array * array_or_not
   | Int of int32
 
+(** Guaranteed equality in terms of both OCaml [(=)] and Javascript [(==)]: if
+    [constant_equal a b = Some v], then [Poly.(=) a b = v] and
+    [Js_of_ocaml.Js.equals a b = v]. This is used for optimization purposes. *)
 val constant_equal : constant -> constant -> bool option
 
 type loc =