[docs] Update auto-subscriptions tutorial to explicitly say that subscribe returns unsubscribe (#6448)

site/content/tutorial/08-stores/02-auto-subscriptions/text.md

@@ -11,6 +11,7 @@ const unsubscribe = count.subscribe(value => {
 	count_value = value;
 });
 ```
+> Calling a `subscribe` method returns an `unsubscribe` function.
 
 You now declared `unsubscribe`, but it still needs to be called, for example through the `onDestroy` [lifecycle hook](tutorial/ondestroy):