[DOC] store,hooks: useState/useStore should return object

or array

closes #404

doc/hooks.md

@@ -181,6 +181,9 @@ class Counter extends owl.Component {
 }
 ```
 
+It is important to remember that `useState` only works with objects or arrays. It
+is necessary, since Owl needs to react to a change in state.
+
 ### `onMounted`
 
 `onMounted` is not a user hook, but is a building block designed to help make useful

doc/store.md

@@ -262,6 +262,10 @@ will only be rerendered whenever this part of the state changes. Otherwise, it
 will perform a strict equality check and will update the component every time this
 check fails.
 
+Also, it may not be obvious, but it is crucial to remember that the selector
+function should return an object or an array.  The reason is that it needs to be
+observed, otherwise the component would not be able to react to changes.
+
 ### `useDispatch`
 
 The `useDispatch` hook is useful when a component needs to be able to dispatch