Extend ProjectJournal docs, thx to @DomClark

include/ProjectJournal.h

@@ -34,6 +34,7 @@
 class JournallingObject;
 
 
+//! @warning many parts of this class may be rewritten soon
 class ProjectJournal
 {
 public:
@@ -76,7 +77,9 @@ public:
 		reallocID( _id, NULL );
 	}
 
+	//! hack, not used when saving a file
 	static jo_id_t idToSave( jo_id_t id );
+	//! hack, not used when loading a savefile
 	static jo_id_t idFromSave( jo_id_t id );
 
 	void clearJournal();

src/core/ProjectJournal.cpp

@@ -29,6 +29,8 @@
 #include "JournallingObject.h"
 #include "Song.h"
 
+//! Avoid clashes between loaded IDs (have the bit cleared)
+//! and newly created IDs (have the bit set)
 static const int EO_ID_MSB = 1 << 23;
 
 const int ProjectJournal::MAX_UNDO_STATES = 100; // TODO: make this configurable in settings