Sync svelte docs (#748)

sync svelte docs

Co-authored-by: Rich-Harris <[email protected]>

Author: github-actions[bot]

apps/svelte.dev/content/docs/svelte/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).

apps/svelte.dev/content/docs/svelte/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.
 

apps/svelte.dev/content/docs/svelte/98-reference/20-svelte.md

@@ -344,7 +344,17 @@ function unmount(component: Record<string, any>): void;
 
 ## untrack
 
-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)
+	});
+});
+```
 
 <div class="ts-block">