wallet: document that tx in CreateTransaction is purely an out-param

src/wallet/wallet.cpp

@@ -3109,13 +3109,14 @@ bool CWallet::CreateTransaction(
         bool sign)
 {
     int nChangePosIn = nChangePosInOut;
-    CTransactionRef tx2 = tx;
+    Assert(!tx); // tx is an out-param. TODO change the return type from bool to tx (or nullptr)
     bool res = CreateTransactionInternal(vecSend, tx, nFeeRet, nChangePosInOut, error, coin_control, fee_calc_out, sign);
     // try with avoidpartialspends unless it's enabled already
     if (res && nFeeRet > 0 /* 0 means non-functional fee rate estimation */ && m_max_aps_fee > -1 && !coin_control.m_avoid_partial_spends) {
         CCoinControl tmp_cc = coin_control;
         tmp_cc.m_avoid_partial_spends = true;
         CAmount nFeeRet2;
+        CTransactionRef tx2;
         int nChangePosInOut2 = nChangePosIn;
         bilingual_str error2; // fired and forgotten; if an error occurs, we discard the results
         if (CreateTransactionInternal(vecSend, tx2, nFeeRet2, nChangePosInOut2, error2, tmp_cc, fee_calc_out, sign)) {