docs: clarify useFormContext formState subscriptions (#1197)

* docs: clarify useFormContext formState subscriptions

* docs: format useFormContext note

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Co-authored-by: hiromieguchi802-lab <hiromieguchi802-lab@users.noreply.github.com>
Co-authored-by: hiromieguchi802-lab <270042125+hiromieguchi802-lab@users.noreply.github.com>

Author: hiromieguchi802-lab

src/content/docs/useformcontext.mdx

@@ -35,6 +35,13 @@ const methods = useFormContext() // retrieve those props
 <Admonition type="important" title="Rules">
   You need to wrap your form with the [`FormProvider`](/docs/formprovider)
   component for `useFormContext` to work properly.
+
+If you need to subscribe to form state values like `errors`, `isDirty`, or
+`dirtyFields` inside a `FormProvider` tree, use
+[`useFormState`](/docs/useformstate) instead of destructuring `formState`
+from `useFormContext()`. `formState` is wrapped with a Proxy, so you should
+read the specific state you want to subscribe to before render.
+
 </Admonition>
 
 **Example:**