rpcwallet: Clarify what settxfee does

src/wallet/rpcwallet.cpp

@@ -2174,7 +2174,7 @@ UniValue settxfee(const UniValue& params, bool fHelp)
     if (fHelp || params.size() < 1 || params.size() > 1)
         throw runtime_error(
             "settxfee amount\n"
-            "\nSet the transaction fee per kB.\n"
+            "\nSet the transaction fee per kB. Overwrites the paytxfee parameter.\n"
             "\nArguments:\n"
             "1. amount         (numeric, required) The transaction fee in " + CURRENCY_UNIT + "/kB rounded to the nearest 0.00000001\n"
             "\nResult\n"