Mention Renaming of useFormState (#65934)

The useFormState hook was renamed to useActionState in react 19. At this
moment react 19 is still in Canary, it would be hard for beginners to
realise this.

Mentioning the fact of it being renamed will ideally help new people.

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Co-authored-by: Sam Ko <sam@vercel.com>

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

@@ -444,6 +444,7 @@ export function Signup() {
 > **Good to know:**
 >
 > - Before mutating data, you should always ensure a user is also authorized to perform the action. See [Authentication and Authorization](#authentication-and-authorization).
+> - In earlier React Canary versions, this API was part of React DOM and called `useFormState`.
 
 #### Optimistic updates