docs: clarify data type in Forms and Mutations section (#54630)

### What?

Clarify the "Forms and Mutations" docs. 

### Why?

This may be opinionated, but I believe this section of the docs starts out assuming that the user knows how the `<form>` HTML works. Coming from the client side React world, most developers are used to using state in forms. Clarifying that this data is accessible via the standard web API [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData/FormData) is essential for both transitioning developers and new developers.

docs/02-app/01-building-your-application/02-data-fetching/03-forms-and-mutations.mdx

@@ -170,6 +170,8 @@ export default function Page() {
 }
 ```
 
+> **Good to know**: `<form action={create}>` takes the [FormData](https://developer.mozilla.org/en-US/docs/Web/API/FormData/FormData) data type. In the example above, the FormData submitted via the HTML [`form`](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/form) is accessible in the server action `create`.
+
 ### Revalidating Data
 
 Server Actions allow you to invalidate the [Next.js Cache](/docs/app/building-your-application/caching) on demand. You can invalidate an entire route segment with [`revalidatePath`](/docs/app/api-reference/functions/revalidatePath):