chore(fundamentals): clarify events (#2992)

docs/core-concepts/fundamentals.md

@@ -45,3 +45,9 @@ Projects such as <a href="https://capacitorjs.com/" target="_blank">Capacitor</a
 ## Theming
 
 At the core, Ionic Framework is built using <a href="https://developer.mozilla.org/en-US/docs/Web/CSS" target="_blank">CSS</a> which allows us to take advantage of the flexibility that <a href="https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables" target="_blank">CSS properties (variables)</a> provide. This makes it incredibly easy to design an app that looks great while following the web standard. We provide a set of colors so developers can have some great defaults, but we encourage overriding them to create designs that match a brand, company or a desired color palette. Everything from the background color of an application to the text color is fully customizable. More information on app theming can be found in [Theming](../theming/basics.md).
+
+## Events
+
+Many Ionic components use [CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent) to inform developers of important state changes in the components. For example, an `ion-datetime` component will emit `ionChange` whenever the selected date has changed.
+
+Developers can use standard events such as `click` as they normally would. However, many events emitted within a component's [shadow root](../reference/glossary.md#shadow) will be [retargeted](https://dom.spec.whatwg.org/#retarget) to the host element. This may result in multiple `click` handlers executing even if the user only clicked once. As a result, developers should rely on Ionic's events to be properly informed of state changes on Ionic components. Ionic's events are prefixed with `ion` to avoid collisions with standard events. Each component's documentation page has a list of available events that developers can listen for in their applications.