### 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{"sendmany",
        "Send multiple times. Amounts are double-precision floating point numbers." +
        HELP_REQUIRING_PASSPHRASE,
                {
                    {"dummy", RPCArg::Type::STR, RPCArg::Default{"\"\""}, "Must be set to \"\" for backwards compatibility.",
                     RPCArgOptions{
                         .oneline_description = "\"\"",
                     }},
                    {"amounts", RPCArg::Type::OBJ_USER_KEYS, RPCArg::Optional::NO, "The addresses and amounts",
                        {
                            {"address", RPCArg::Type::AMOUNT, RPCArg::Optional::NO, "The bitcoin address is the key, the numeric amount (can be string) in " + CURRENCY_UNIT + " is the value"},
                        },
                    },
                    {"minconf", RPCArg::Type::NUM, RPCArg::Optional::OMITTED, "Ignored dummy value"},
                    {"comment", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "A comment"},
                    {"subtractfeefrom", RPCArg::Type::ARR, RPCArg::Optional::OMITTED, "The addresses.\n"
                                       "The fee will be equally deducted from the amount of each selected address.\n"
                                       "Those recipients will receive less bitcoins than you enter in their corresponding amount field.\n"
                                       "If no addresses are specified here, the sender pays the fee.",
                        {
                            {"address", RPCArg::Type::STR, RPCArg::Optional::OMITTED, "Subtract fee from this address"},
                        },
                    },
                    {"replaceable", RPCArg::Type::BOOL, RPCArg::DefaultHint{"wallet default"}, "Signal that this transaction can be replaced by a transaction (BIP 125)"},
                    {"conf_target", RPCArg::Type::NUM, RPCArg::DefaultHint{"wallet -txconfirmtarget"}, "Confirmation target in blocks"},