### 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_
{ "lockunspent", 1, "transactions" },
    { "lockunspent", 2, "persistent" },
    { "send", 0, "outputs" },
    { "send", 1, "conf_target" },
    { "send", 3, "fee_rate"},
    { "send", 4, "options" },
    { "send", 4, "add_inputs"},
    { "send", 4, "include_unsafe"},
    { "send", 4, "minconf"},
    { "send", 4, "maxconf"},
    { "send", 4, "add_to_wallet"},
    { "send", 4, "change_position"},
    { "send", 4, "fee_rate"},
    { "send", 4, "include_watching"},
    { "send", 4, "inputs"},
    { "send", 4, "locktime"},
    { "send", 4, "lock_unspents"},
    { "send", 4, "psbt"},
    { "send", 4, "subtract_fee_from_outputs"},
    { "send", 4, "conf_target"},
    { "send", 4, "replaceable"},
    { "send", 4, "solving_data"},
    { "sendall", 0, "recipients" },
    { "sendall", 1, "conf_target" },
    { "sendall", 3, "fee_rate"},
    { "sendall", 4, "options" },
    { "sendall", 4, "add_to_wallet"},
    { "sendall", 4, "fee_rate"},
    { "sendall", 4, "include_watching"},
    { "sendall", 4, "inputs"},