Document "gotcha" regarding `dispatch`.

The added remark might help people to save some time.

site/content/tutorial/05-events/04-component-events/text.md

@@ -18,4 +18,5 @@ Components can also dispatch events. To do so, they must create an event dispatc
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 ```
 
-> `createEventDispatcher` must be called when the component is first instantiated — you can't do it later inside e.g. a `setTimeout` callback. This links `dispatch` to the component instance.
+> `createEventDispatcher` must be called when the component is first instantiated — you can't do it later inside e.g. a `setTimeout` callback. This links `dispatch` to the component instance.
+> `dispatch` can only be called once the component has been mounted. It is therefore not possible to call `dispatch` from the `beforeUpdate` lifecycle.