[onnx] Small doc fixup.

lib/onnx/onnx.mli

@@ -27,7 +27,7 @@ type t = private {
   n_outputs : int;  (** Number of outputs. *)
   nier : (G.t, string) Result.t;  (** Intermediate representation. *)
 }
-(** ONNX model metadata. *)
+(** ONNX model metadata and intermediate representation. *)
 
 val parse : string -> (t, string) Result.t
 (** Parse an ONNX file. *)