splice: add "for now" to "Don't support wallet funding being used for fee"

CLN `v25.05`

Did a splice with a script and it was easy and worked beautifully!

However, I was very confused by the error message when trying to add a fee (rate) like in the [docs](https://docs.corelightning.org/reference/dev-splice):

```
lightning-cli dev-splice script_or_json="wallet -> 37602sat+fee@2000; 37602sat -> 00e8fa5f98228486a276abaa8e970d3b8433ff8afc7950f78973e5ead9078124" dryrun=true
{
"code": -32602,
"message": "Don't support wallet funding being used for fee"
}
```

a "for now" would have cleared it all up.

In ` plugins/spender/splice.c` around line 1285

add "for now" **also** to:

```
"Don't support wallet funding"
" being used for fee");
```

and possibly somewhere else.

BTW: these messages are easier to find in the code (for example with `ag`) if they are on the same line:
```
"Don't support wallet funding being used for fee");
```