### Is there an existing issue for this?

- [X] I have searched the existing issues

### Current behaviour

See inline discussion at https://github.com/bitcoin/bitcoin/pull/26467#discussion_r1269177446

@whitslack:

> Consider the case where I am paying someone on chain under the stipulation that _they_ will eat the mining fee. (Thus, I specify _their_ address in `subtractfeefrom` when I'm constructing my transaction.) Later, they tell me that they can't wait for confirmation any longer and need an urgent fee bump. If I naïvely specify their output as the `reduce_output`, I may end up paying them _**much more**_ than I intended to if I only have large UTxOs in my wallet.


### Expected behaviour

Not sure.

Improving the docs is one option, but this behaviour seem quite counter intuitive.

My own suggestion:

> Should we disable the adding of new inputs when `reduce_output` is set? If someone really intends to do that, they should probably use `outputs`.

### Steps to reproduce

Haven't tried.

### Relevant log output

_No response_

### How did you obtain Bitcoin Core

Compiled from source

### What version of Bitcoin Core are you using?

Master after #26467

### Operating system and version

Any

### Machine specifications

_No response_
RPCExamples{
    "\nBump the fee, get the new transaction\'s " + std::string(want_psbt ? "psbt" : "txid") + "\n" +
            HelpExampleCli(method_name, "<txid>")
        },
        [want_psbt](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue
{
    std::shared_ptr<CWallet> const pwallet = GetWalletForJSONRPCRequest(request);
    if (!pwallet) return UniValue::VNULL;
    if (pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS) && !pwallet->IsWalletFlagSet(WALLET_FLAG_EXTERNAL_SIGNER) && !want_psbt) {
        throw JSONRPCError(RPC_WALLET_ERROR, "bumpfee is not available with wallets that have private keys disabled. Use psbtbumpfee instead.");
    }
    uint256 hash(ParseHashV(request.params[0], "txid"));
    CCoinControl coin_control;
    coin_control.fAllowWatchOnly = pwallet->IsWalletFlagSet(WALLET_FLAG_DISABLE_PRIVATE_KEYS);
    
    coin_control.m_signal_bip125_rbf = true;
    std::vector<CTxOut> outputs;
    std::optional<uint32_t> reduce_output;
    if (!request.params[1].isNull()) {
        UniValue options = request.params[1];
        RPCTypeCheckObj(options,
            {
                {"confTarget", UniValueType(UniValue::VNUM)},
                {"conf_target", UniValueType(UniValue::VNUM)},
                {"fee_rate", UniValueType()}, 
                {"replaceable", UniValueType(UniValue::VBOOL)},
                {"estimate_mode", UniValueType(UniValue::VSTR)},
                {"outputs", UniValueType()}, 
                {"reduce_output", UniValueType(UniValue::VNUM)},
            },