Customizable `<select multiple>` rendering dropdown

### MDN URL

https://developer.mozilla.org/en-US/docs/Learn_web_development/Extensions/Forms/Customizable_select

### What specific section or headline is this issue about?

_No response_

### What information was incorrect, unhelpful, or incomplete?

We are implementing base appearance for the last mode of the select element, the multiple drop down: ``
- https://github.com/whatwg/html/pull/12069
- https://chromestatus.com/feature/5070067395067904

Here is a basic example:
```html

one
two

select, ::picker(select) {
appearance: base-select;
}

```

Base appearance for single-select dropdown was already documented here: https://github.com/mdn/content/issues/37857

Base appearance for single and multi-select listboxes is tracked here: https://github.com/mdn/content/issues/42358

Related issue about mobile/desktop parity: https://github.com/mdn/content/issues/43403

### What did you expect to see?

Documentation about using appearance:base-select on ``

### Do you have any supporting links, references, or citations?

_No response_

### Do you have anything more you want to share?

I mentioned in the [mobile/desktop parity issue](https://github.com/mdn/content/issues/43403) that the spec was written such that `` may be rendered as either a listbox or dropdown in some browsers. However, with the way discussions have been going so far, putting appearance:base-select on `` is supposed to guarantee that it will be a drop-down box.