update onMount docs

Author: benmccann

packages/svelte/src/index-client.js

@@ -17,6 +17,9 @@ import { legacy_mode_flag } from './internal/flags/index.js';
  *
  * `onMount` does not run inside [server-side components](https://svelte.dev/docs/svelte/svelte-server#render).
  *
+ * `onMount` runs only once. If you need reactivity, you can use [`$effect`](https://svelte.dev/docs/svelte/$effect),
+ * which also runs when the component is mounted to the DOM.
+ * 
  * @template T
  * @param {() => NotFunction<T> | Promise<NotFunction<T>> | (() => any)} fn
  * @returns {void}

packages/svelte/types/index.d.ts

@@ -356,6 +356,9 @@ declare module 'svelte' {
 	 *
 	 * `onMount` does not run inside [server-side components](https://svelte.dev/docs/svelte/svelte-server#render).
 	 *
+	 * `onMount` runs only once. If you need reactivity, you can use [`$effect`](https://svelte.dev/docs/svelte/$effect),
+	 * which also runs when the component is mounted to the DOM.
+	 *
 	 * */
 	export function onMount<T>(fn: () => NotFunction<T> | Promise<NotFunction<T>> | (() => any)): void;
 	/**