### 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_
#include <common/args.h>
#include <rpc/client.h>
#include <tinyformat.h>
#include <set>
#include <stdint.h>
#include <string>
#include <string_view>
class CRPCConvertParam
{
public:
    std::string methodName; 
    int paramIdx;           
    std::string paramName;  
};
/**
 * Specify a (method, idx, name) here if the argument is a non-string RPC
 * argument and needs to be converted from JSON.
 *
 * @note Parameter indexes start from 0.
 */
static const CRPCConvertParam vRPCConvertParams[] =
{
    { "setmocktime", 0, "timestamp" },
    { "mockscheduler", 0, "delta_time" },
    { "utxoupdatepsbt", 1, "descriptors" },
    { "generatetoaddress", 0, "nblocks" },