tests for transaction RAII behaviour, transaction documentation, minor doc fixes

src/transaction.h

@@ -23,9 +23,9 @@ protected:
     Transaction(TransactionID txn, const Base* parent);
 
 protected:
-    TransactionID txn;
-    bool active;
-    const Base* parent;
+    TransactionID txn;          /**<\brief Transaction inner handler*/
+    bool active;                /**<\brief Transaction state*/
+    const Base* parent;         /**<\brief Pointer to the database this transaction belongs to*/
 };
 
 class WriteTransaction : public Transaction {