Address Xavier's review

stdlib/random.mli

@@ -43,8 +43,13 @@ val int : int -> int
      than 2{^30}. *)
 
 val int63 : int -> int
-(** [Random.int63 bound] is identical to {!int} if [bound] is less than 2{^30},
-    however [bound] may be any positive [int].
+(** [Random.int63 bound] returns a random integer between 0 (inclusive)
+     and [bound] (exclusive). [bound] may be any positive integer.
+
+     If [bound] is less than 2{^30}, [Random.int63 bound] is equal to
+     {!Random.int}[ bound]. If [bound] is greater than 2{^30} (on 64-bit systems
+     or non-standard environments, such as JavaScript), [Random.int63] returns
+     a value, where {!Random.int} raises {!Invalid_argument}.
 
     @since 4.12.0 *)