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

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 ref `useState` creates will be persisted from server to client, it is important that it does not contain anything (on server side) that cannot be serialized to JSON, such as a class or function. Once loaded on the client, it behaves as a normal ref and does not have this limitation.
+::
+
 ::ReadMore{link="/guide/features/state-management"}
 ::