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/wallet/test/wallet_tests.cpp

@@ -558,7 +558,7 @@ public:
         CTransactionRef tx;
         CCoinControl dummy;
         {
-            auto res = CreateTransaction(*wallet, {recipient}, std::nullopt, dummy);
+            auto res = CreateTransaction(*wallet, {recipient}, /*change_pos=*/std::nullopt, dummy);
             BOOST_CHECK(res);
             tx = res->tx;
         }