Add a backup warning to the encryptwallet RPC command

src/rpcwallet.cpp

@@ -1396,7 +1396,7 @@ Value encryptwallet(const Array& params, bool fHelp)
     // slack space in .dat files; that is bad if the old data is
     // unencrypted private keys. So:
     StartShutdown();
-    return "wallet encrypted; Bitcoin server stopping, restart to run with encrypted wallet";
+    return "wallet encrypted; Bitcoin server stopping, restart to run with encrypted wallet.  The keypool has been flushed, you need to make a new backup.";
 }
 
 class DescribeAddressVisitor : public boost::static_visitor<Object>