[Doc] Add Doxygen docstrings for wrapper classes

Without at least a basic docstring, these classes are excluded from the Doxygen
documentation.

include/cantera/Edge.h

@@ -9,6 +9,7 @@
 namespace Cantera
 {
 
+//! Convenience class which inherits from both EdgePhase and EdgeKinetics
 class Edge :
     public EdgePhase, public EdgeKinetics
 {

include/cantera/IdealGasMix.h

@@ -10,6 +10,7 @@
 namespace Cantera
 {
 
+//! Convenience class which inherits from both IdealGasPhase and GasKinetics
 class IdealGasMix :
     public IdealGasPhase,
     public GasKinetics

include/cantera/IncompressibleSolid.h

@@ -8,6 +8,7 @@
 namespace Cantera
 {
 
+//! Wrapper for ConstDensityThermo with constructor from file
 class IncompressibleSolid : public ConstDensityThermo
 {
 public:

include/cantera/Metal.h

@@ -8,6 +8,7 @@
 namespace Cantera
 {
 
+//! Wrapper for MetalPhase with constructor from file
 class Metal : public MetalPhase
 {
 public:

include/cantera/PureFluid.h

@@ -8,6 +8,7 @@
 namespace Cantera
 {
 
+//! Wrapper for PureFluidPhase with constructor from file
 class PureFluid : public PureFluidPhase
 {
 public: