consensus/doc: explain unreachable bad-txns-fee-outofrange check

After the previous conditions were validated, document why `bad-txns-fee-outofrange` is unreachable once `nValueIn` and `value_out` are `MoneyRange` and `nValueIn >= value_out`.

Although unreachable, keep the check itself in place (instead of removing) as it's part of consensus-critical code; the comment serves as a proof for future refactors.

Inspired by b-c-cov coverage reports:
* "bad-txns-fee-outofrange" - https://maflcko.github.io/b-c-cov/test_bitcoin.coverage/src/consensus/tx_verify.cpp.gcov.html#L200

src/consensus/tx_verify.cpp

@@ -197,6 +197,11 @@ bool Consensus::CheckTxInputs(const CTransaction& tx, TxValidationState& state,
     // Tally transaction fees
     const CAmount txfee_aux = nValueIn - value_out;
     if (!MoneyRange(txfee_aux)) {
+        // Unreachable, given the following preconditions:
+        // * `value_out` comes from `tx.GetValueOut()`, which throws unless `MoneyRange(value_out)` and asserts `MoneyRange(nValueOut)` on return.
+        // * `MoneyRange(nValueIn)` was enforced in the input loop.
+        // * `nValueIn < value_out` was handled above, so `nValueIn >= value_out` here (and `txfee_aux >= 0`).
+        // Therefore `0 <= txfee_aux = nValueIn - value_out <= nValueIn <= MAX_MONEY`.
         return state.Invalid(TxValidationResult::TX_CONSENSUS, "bad-txns-fee-outofrange");
     }