test: wallet, fix change position out of range error

Since #25273, the behavior of 'inserting change at a random
position' is instructed by passing std::nullopt instead of -1.

Also, added missing documentation about the meaning of
'change_pos=std::nullopt' inside 'CWallet::CreateTransaction()'

src/bench/wallet_create_tx.cpp

@@ -129,7 +129,7 @@ static void WalletCreateTx(benchmark::Bench& bench, const OutputType output_type
 
     bench.epochIterations(5).run([&] {
         LOCK(wallet.cs_wallet);
-        const auto& tx_res = CreateTransaction(wallet, recipients, -1, coin_control);
+        const auto& tx_res = CreateTransaction(wallet, recipients, /*change_pos=*/std::nullopt, coin_control);
         assert(tx_res);
     });
 }