update onMount docs

Author: Rich-Harris

packages/svelte/src/index-client.js

@@ -9,16 +9,14 @@ import { lifecycle_outside_component } from './internal/shared/errors.js';
 import { legacy_mode_flag } from './internal/flags/index.js';
 
 /**
- * The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM.
- * It must be called during the component's initialisation (but doesn't need to live *inside* the component;
- * it can be called from an external module).
+ * `onMount`, like [`$effect`](https://svelte.dev/docs/svelte/$effect), schedules a function to run as soon as the component has been mounted to the DOM.
+ * Unlike `$effect`, the provided function only runs once.
  *
- * If a function is returned _synchronously_ from `onMount`, it will be called when the component is unmounted.
+ * It must be called during the component's initialisation (but doesn't need to live _inside_ the component;
+ * it can be called from an external module). If a function is returned _synchronously_ from `onMount`,
+ * it will be called when the component is unmounted.
  *
- * `onMount` does not run inside [server-side components](https://svelte.dev/docs/svelte/svelte-server#render).
- *
- * `onMount` runs only once. If you need reactivity, you can use [`$effect`](https://svelte.dev/docs/svelte/$effect),
- * which also runs when the component is mounted to the DOM.
+ * `onMount` functions do not run during [server-side rendering](https://svelte.dev/docs/svelte/svelte-server#render).
  *
  * @template T
  * @param {() => NotFunction<T> | Promise<NotFunction<T>> | (() => any)} fn

packages/svelte/types/index.d.ts

@@ -348,16 +348,14 @@ declare module 'svelte' {
 				props: Props;
 			});
 	/**
-	 * The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM.
-	 * It must be called during the component's initialisation (but doesn't need to live *inside* the component;
-	 * it can be called from an external module).
+	 * `onMount`, like [`$effect`](https://svelte.dev/docs/svelte/$effect), schedules a function to run as soon as the component has been mounted to the DOM.
+	 * Unlike `$effect`, the provided function only runs once.
 	 *
-	 * If a function is returned _synchronously_ from `onMount`, it will be called when the component is unmounted.
+	 * It must be called during the component's initialisation (but doesn't need to live _inside_ the component;
+	 * it can be called from an external module). If a function is returned _synchronously_ from `onMount`,
+	 * it will be called when the component is unmounted.
 	 *
-	 * `onMount` does not run inside [server-side components](https://svelte.dev/docs/svelte/svelte-server#render).
-	 *
-	 * `onMount` runs only once. If you need reactivity, you can use [`$effect`](https://svelte.dev/docs/svelte/$effect),
-	 * which also runs when the component is mounted to the DOM.
+	 * `onMount` functions do not run during [server-side rendering](https://svelte.dev/docs/svelte/svelte-server#render).
 	 *
 	 * */
 	export function onMount<T>(fn: () => NotFunction<T> | Promise<NotFunction<T>> | (() => any)): void;