Fix comment about how wallet txs are sorted

The wallet transactions in the node are not sorted
by hash. Also make comment Doxygen compatible.

src/qt/transactiontablemodel.cpp

@@ -93,10 +93,7 @@ public:
 
     TransactionTableModel *parent;
 
-    /* Local cache of wallet.
-     * As it is in the same order as the CWallet, by definition
-     * this is sorted by sha256.
-     */
+    //! Local cache of wallet sorted by transaction hash
     QList<TransactionRecord> cachedWallet;
 
     /** True when model finishes loading all wallet transactions on start */