docs: tweak untrack description, provide an example of usage (#14085)

* docs: tweak untrack description, provide an example of usage

* link to untrack

* add derived docs too

Author: Rich-Harris

documentation/docs/02-runes/03-$derived.md

@@ -45,3 +45,9 @@ Sometimes you need to create complex derivations that don't fit inside a short e
 ```
 
 In essence, `$derived(expression)` is equivalent to `$derived.by(() => expression)`.
+
+## Understanding dependencies
+
+Anything read synchronously inside the `$derived` expression (or `$derived.by` function body) is considered a _dependency_ of the derived state. When the state changes, the derived will be marked as _dirty_ and recalculated when it is next read.
+
+To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).

documentation/docs/02-runes/04-$effect.md

@@ -2,7 +2,7 @@
 title: $effect
 ---
 
-Effects are what make your application _do things_. When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed, and re-runs the function when that state later changes.
+Effects are what make your application _do things_. When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed (unless accessed inside [`untrack`](svelte#untrack)), and re-runs the function when that state later changes.
 
 Most of the effects in a Svelte app are created by Svelte itself — they're the bits that update the text in `<h1>hello {name}!</h1>` when `name` changes, for example.
 

packages/svelte/src/internal/client/runtime.js

@@ -831,7 +831,17 @@ export function invalidate_inner_signals(fn) {
 }
 
 /**
- * Use `untrack` to prevent something from being treated as an `$effect`/`$derived` dependency.
+ * When used inside a [`$derived`](https://svelte.dev/docs/svelte/$derived) or [`$effect`](https://svelte.dev/docs/svelte/$effect),
+ * any state read inside `fn` will not be treated as a dependency.
+ *
+ * ```ts
+ * $effect(() => {
+ *   // this will run when `data` changes, but not when `time` changes
+ *   save(data, {
+ *     timestamp: untrack(() => time)
+ *   });
+ * });
+ * ```
  * @template T
  * @param {() => T} fn
  * @returns {T}

packages/svelte/types/index.d.ts

@@ -455,7 +455,17 @@ declare module 'svelte' {
 	 * */
 	export function tick(): Promise<void>;
 	/**
-	 * Use `untrack` to prevent something from being treated as an `$effect`/`$derived` dependency.
+	 * When used inside a [`$derived`](https://svelte.dev/docs/svelte/$derived) or [`$effect`](https://svelte.dev/docs/svelte/$effect),
+	 * any state read inside `fn` will not be treated as a dependency.
+	 *
+	 * ```ts
+	 * $effect(() => {
+	 *   // this will run when `data` changes, but not when `time` changes
+	 *   save(data, {
+	 *     timestamp: untrack(() => time)
+	 *   });
+	 * });
+	 * ```
 	 * */
 	export function untrack<T>(fn: () => T): T;
 	/**