[newton] Improve doc of apply

dumux/nonlinear/newtonsolver.hh

@@ -369,6 +369,8 @@ public:
      *        The solver is responsible for all the strategic decisions.
      * \param vars The variables object representing the current state of the
      *             numerical solution (primary and possibly secondary variables).
+     * \post If converged, the `Variables` will represent the solution. If convergence
+     *       fails, they are in some intermediate, undefined state.
      */
     bool apply(Variables& vars) override
     {