### 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_
{
    return RPCHelpMan{"settxfee",
                "\nSet the transaction fee rate in " + CURRENCY_UNIT + "/kvB for this wallet. Overrides the global -paytxfee command line parameter.\n"
                "Can be deactivated by passing 0 as the fee. In that case automatic fee selection will be used by default.\n",
                {
                    {"amount", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "The transaction fee rate in " + CURRENCY_UNIT + "/kvB"},
                },
                RPCResult{
                    RPCResult::Type::BOOL, "", "Returns true if successful"
                },
                RPCExamples{
                    HelpExampleCli("settxfee", "0.00001")
            + HelpExampleRpc("settxfee", "0.00001")
                },
        [&](const RPCHelpMan& self, const JSONRPCRequest& request) -> UniValue