Docs: Note $ACTION_ formData properties (#63048)

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### What?

Make a note about the new `$ACTION_` properties in the `formData`

### Why?

Since `[email protected]`, some new `$ACTION_` properties have appeared in
the`formData`, which cause validation errors when used with a strict
validation with a library such as Zod or Yup:

```tsx
// Form Client Component
export default function Form() {
  const [state, formAction] = useFormState(action, {
    type: 'initial',
  });

  return (
    <form action={action}>
      {/* ... */}
    </form>
  );
}

// Server Action
export async function action(
  prevState: ServerActionReturnValue,
  formData: FormData,
): Promise<ServerActionReturnValue> {
  const validationResult = await schema.validate(
    Object.fromEntries(formData.entries()),
  );
```

The errors from Yup:

```
Errors:

- this field has unspecified keys: $ACTION_REF_1, $ACTION_1:0, $ACTION_1:1, $ACTION_KEY
```

The data in the `formData.entries()`

```js
{
  '$ACTION_REF_1': '',
  '$ACTION_1:0': '{"id":"59f475b...","bound":"$@1"}',
  '$ACTION_1:1': '[{"type":"initial"}]',
  '$ACTION_KEY': 'k187677481',
  // Real form data starts here:
  year: '2024',
```

### How?

Document additional properties with keys starting with the prefix
`$ACTION_`

### Alternatives Considered

Maybe these `$ACTION_` properties are not intended to be exposed to
users, and should be stripped before the `formData` reaches the Server
Action.

### Related

The documentation was originally introduced in commit
[`687239c`](687239c),
which was a part of this PR:

- #59080

cc @delbaoliveira

---------

Co-authored-by: Sam Ko <[email protected]>

Author: karlhorky

docs/02-app/01-building-your-application/02-data-fetching/02-server-actions-and-mutations.mdx

@@ -171,7 +171,7 @@ export default function Page() {
 > **Good to know:**
 >
 > - Example: [Form with Loading & Error States](https://github.com/vercel/next.js/tree/canary/examples/next-forms)
-> - When working with forms that have many fields, you may want to consider using the [`entries()`](https://developer.mozilla.org/en-US/docs/Web/API/FormData/entries) method with JavaScript's [`Object.fromEntries()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/fromEntries). For example: `const rawFormData = Object.fromEntries(formData.entries())`
+> - When working with forms that have many fields, you may want to consider using the [`entries()`](https://developer.mozilla.org/en-US/docs/Web/API/FormData/entries) method with JavaScript's [`Object.fromEntries()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/fromEntries). For example: `const rawFormData = Object.fromEntries(formData.entries())`. One thing to note is that the `formData` will include additional `$ACTION_`properties.
 > - See [React `<form>` documentation](https://react.dev/reference/react-dom/components/form#handle-form-submission-with-a-server-action) to learn more.
 
 #### Passing Additional Arguments