Add a few class documentations to override inherited doc.

clang doxygen comments follows class inheritance.
Undocumented Ardour classes which inherit from sigc::trackable
also inherit sigc's documentation.

libs/ardour/ardour/export_handler.h

@@ -72,6 +72,7 @@ class LIBARDOUR_API ExportElementFactory
 	Session & session;
 };
 
+/** Export Handler */
 class LIBARDOUR_API ExportHandler : public ExportElementFactory, public sigc::trackable
 {
   public:

libs/ardour/ardour/location.h

@@ -41,6 +41,7 @@ namespace ARDOUR {
 
 class SceneChange;
 
+/** Location on Timeline - abstract representation for Markers, Loop/Punch Ranges, CD-Markers etc. */
 class LIBARDOUR_API Location : public SessionHandleRef, public PBD::StatefulDestructible
 {
   public:

libs/ardour/ardour/session.h

@@ -161,6 +161,7 @@ private:
 	std::string _message;
 };
 
+/** Ardour Session */
 class LIBARDOUR_API Session : public PBD::StatefulDestructible, public PBD::ScopedConnectionList, public SessionEventManager
 {
   public:

libs/ardour/ardour/tempo.h

@@ -212,6 +212,9 @@ class LIBARDOUR_API TempoMetric {
 	Timecode::BBT_Time _start;
 };
 
+/** Tempo Map - mapping of timecode to musical time.
+ * convert audio-samples, sample-rate to Bar/Beat/Tick, Meter/Tempo
+ */
 class LIBARDOUR_API TempoMap : public PBD::StatefulDestructible
 {
   public:

libs/pbd/pbd/statefuldestructible.h

@@ -26,6 +26,7 @@
 
 namespace PBD {
 
+/** Base class for objects with saveable and undoable state with destruction notification */
 class LIBPBD_API StatefulDestructible : public Stateful, public Destructible
 {
 };