added comment about the validity range of the saturated vapor pressure curve according to IAPWS

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@12088 2fb0f335-1f38-0410-981e-8018bf24f1b0

dumux/material/components/iapws/region4.hh

@@ -63,7 +63,8 @@ public:
      *
      *\param temperature temperature of component in \f$\mathrm{[K]}\f$
      *
-     * The saturation pressure is often also called vapor pressure.
+     * The saturation pressure is often also called vapor pressure. This formulation is valid
+     * for a temperature range between 273.15 K and 647.096 K (critical temperature).
      */
     static Scalar saturationPressure(Scalar temperature)
     {