docs: note custom elements on caveat (#14934)

closes #14727

Author: dummdidumm

documentation/docs/07-misc/04-custom-elements.md

@@ -125,3 +125,4 @@ Custom elements can be a useful way to package components for consumption in a n
 - The deprecated `let:` directive has no effect, because custom elements do not have a way to pass data to the parent component that fills the slot
 - Polyfills are required to support older browsers
 - You can use Svelte's context feature between regular Svelte components within a custom element, but you can't use them across custom elements. In other words, you can't use `setContext` on a parent custom element and read that with `getContext` in a child custom element.
+- Don't declare properties or attributes starting with `on`, as their usage will be interpreted as an event listener. In other words, Svelte treats `<custom-element oneworld={true}></custom-element>` as `customElement.addEventListener('eworld', true)` (and not as `customElement.oneworld = true`)