Updated usage message

main.ml

@@ -50,7 +50,7 @@ let main () =
         ("-o", Arg.String (fun s -> args.outfile <- Some s),
             "Output file (defaults to foo.s for foo.cvc)");
         ("-v", Arg.Int (fun i -> args.verbose <- i),
-            "Set verbosity (0|1|2)");
+            "Set verbosity (0: nothing, 1: phase titles, 2: intermediate, 3: debug)");
         ("-nocpp", Arg.Unit (fun _ -> args.cpp <- false),
             "Disable C preprocessor");
         ("-noopt", Arg.Unit (fun _ -> args.optimize <- false),