[doc/MPL] Add missing IdealGasLaw tag docu.

Documentation/ProjectFile/properties/property/IdealGasLaw/c_IdealGasLaw.md

+\copydoc MaterialPropertyLib::IdealGasLaw

MaterialLib/MPL/Properties/CreateIdealGasLaw.cpp

@@ -21,6 +21,7 @@ std::unique_ptr<IdealGasLaw> createIdealGasLaw(
     //! \ogs_file_param{properties__property__type}
     config.checkConfigParameter("type", "IdealGasLaw");
     DBUG("Create IdealGasLaw medium property");
+    //! \ogs_file_param_special{properties__property__IdealGasLaw}
     return std::make_unique<IdealGasLaw>();
 }
 }  // namespace MaterialPropertyLib

MaterialLib/MPL/Properties/IdealGasLaw.h

@@ -23,11 +23,10 @@ class Medium;
 class Phase;
 class Component;
 /**
- * \class IdealGasLaw
- * \brief Density function for ideal gases
+ * \brief Density function for ideal gases.
  * \details This property must be either a phase or a component property, it
  * computes the density of an ideal gas as function of phase pressure and
- * temperature
+ * temperature.
  */
 class IdealGasLaw final : public Property
 {