docs: warn that useState should only contain JSON-serializable content

docs/content/2.guide/2.features/6.state-management.md

@@ -10,6 +10,9 @@ Nuxt provides `useState` composable to create a reactive and SSR-friendly shared
 ::alert{icon=👉}
 `useState` only works during `setup` or [`Lifecycle Hooks`](https://vuejs.org/api/composition-api-lifecycle.html#composition-api-lifecycle-hooks).
 ::
+::alert{type=warning}
+Because the data inside `useState` will be persisted from server to client, it is important that it does not contain anything that cannot be serialized to JSON, such as a class or function.
+::
 
 ## Best practices
 

docs/content/3.api/1.composables/use-state.md

@@ -9,5 +9,9 @@ useState<T>(key: string, init?: () => T | Ref<T>): Ref<T>
 * **init**: A function that provides initial value for the state when not initiated. This function can also return a `Ref`.
 * **T**: (typescript only) Specify the type of state
 
+::alert{type=warning}
+Because the data inside `useState` will be persisted from server to client, it is important that it does not contain anything that cannot be serialized to JSON, such as a class or function.
+::
+
 ::ReadMore{link="/guide/features/state-management"}
 ::