Add note about $ACTION_ properties, remove reference to .entries() (#84459)

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### What?

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Closes NEXT-
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### What?

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

### Why?

Starting with [email protected], `$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?

1. Document additional properties with keys starting with the prefix
`$ACTION_`
2. Remove the obsolete mention of `.entries()`, since that is not being
used in the code example anymore (previously, it was `const rawFormData
= Object.fromEntries(formData.entries())`)

### History

- The `$ACTION_` property change was added in
#63048
- The `docs/01-app/02-guides/forms.mdx` file was added in PR
#79288 , without the `$ACTION_`
property change

cc @delbaoliveira

Co-authored-by: Joseph <[email protected]>

Author: karlhorky

docs/01-app/02-guides/forms.mdx

@@ -50,7 +50,7 @@ export default function Page() {
 }
 ```
 
-> **Good to know:** When working with forms that have multiple fields, you can use 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)`.
+> **Good to know:** When working with forms that have multiple fields, use 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)`. Note that this object will contain extra properties prefixed with `$ACTION_`.
 
 ## Passing additional arguments