### 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{"walletcreatefundedpsbt",
                "\nCreates and funds a transaction in the Partially Signed Transaction format.\n"
                "Implements the Creator and Updater roles.\n"
                "All existing inputs must either have their previous output transaction be in the wallet\n"
                "or be in the UTXO set. Solving data must be provided for non-wallet inputs.\n",
                {
                    {"inputs", RPCArg::Type::ARR, RPCArg::Optional::OMITTED, "Leave empty to add inputs automatically. See add_inputs option.",
                        {
                            {"", RPCArg::Type::OBJ, RPCArg::Optional::OMITTED, "",
                                {
                                    {"txid", RPCArg::Type::STR_HEX, RPCArg::Optional::NO, "The transaction id"},
                                    {"vout", RPCArg::Type::NUM, RPCArg::Optional::NO, "The output number"},
                                    {"sequence", RPCArg::Type::NUM, RPCArg::DefaultHint{"depends on the value of the 'locktime' and 'options.replaceable' arguments"}, "The sequence number"},
                                    {"weight", RPCArg::Type::NUM, RPCArg::DefaultHint{"Calculated from wallet and solving data"}, "The maximum weight for this input, "
                                        "including the weight of the outpoint and sequence number. "
                                        "Note that signature sizes are not guaranteed to be consistent, "
                                        "so the maximum DER signatures size of 73 bytes should be used when considering ECDSA signatures."
                                        "Remember to convert serialized sizes to weight units when necessary."},
                                },
                            },
                        },
                        },
                    {"outputs", RPCArg::Type::ARR, RPCArg::Optional::NO, "The outputs specified as key-value pairs.\n"