### 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_
{ "walletcreatefundedpsbt", 3, "includeWatching"},
    { "walletcreatefundedpsbt", 3, "lockUnspents"},
    { "walletcreatefundedpsbt", 3, "fee_rate"},
    { "walletcreatefundedpsbt", 3, "feeRate"},
    { "walletcreatefundedpsbt", 3, "subtractFeeFromOutputs"},
    { "walletcreatefundedpsbt", 3, "conf_target"},
    { "walletcreatefundedpsbt", 3, "replaceable"},
    { "walletcreatefundedpsbt", 3, "solving_data"},
    { "walletcreatefundedpsbt", 4, "bip32derivs" },
    { "walletprocesspsbt", 1, "sign" },
    { "walletprocesspsbt", 3, "bip32derivs" },
    { "walletprocesspsbt", 4, "finalize" },
    { "descriptorprocesspsbt", 1, "descriptors"},
    { "descriptorprocesspsbt", 3, "bip32derivs" },
    { "descriptorprocesspsbt", 4, "finalize" },
    { "createpsbt", 0, "inputs" },
    { "createpsbt", 1, "outputs" },
    { "createpsbt", 2, "locktime" },
    { "createpsbt", 3, "replaceable" },
    { "combinepsbt", 0, "txs"},
    { "joinpsbts", 0, "txs"},
    { "finalizepsbt", 1, "extract"},
    { "converttopsbt", 1, "permitsigdata"},
    { "converttopsbt", 2, "iswitness"},
    { "gettxout", 1, "n" },
    { "gettxout", 2, "include_mempool" },
    { "gettxoutproof", 0, "txids" },
    { "gettxoutsetinfo", 1, "hash_or_height" },
    { "gettxoutsetinfo", 2, "use_index"},
    { "lockunspent", 0, "unlock" },