Update documentation about output_concrete_model

I also remove the mention about the different kind of formats used
to print models. Indeed, we use only the SMT-LIB format and
the Why3 format is slightly different but probably outdated.

src/lib/frontend/models.mli

@@ -37,10 +37,6 @@ type t = {
   arrays : ModelMap.t;
 }
 
-(** Print the given counterexample on the given formatter with the
-    corresponding format set with Options.get_output_format.
-    - functions: the functions of the model;
-    - constants: the variables of the model;
-    - arrays: (experimental) the arrays of the model.
-*)
 val output_concrete_model : Format.formatter -> t -> unit
+(** [output_concrete_model ppf mdl] prints the model [mdl] on
+    the formatter [ppf]. *)