message: wallet: document that tx in CreateTransaction is purely an out-param
diff: @@ -3140,13 +3140,14 @@ 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)) {
  