docs: add section on update propagation (#14938)

Author: Rich-Harris

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

@@ -51,3 +51,20 @@ In essence, `$derived(expression)` is equivalent to `$derived.by(() => expressio
 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).
+
+## Update propagation
+
+Svelte uses something called _push-pull reactivity_ — when state is updated, everything that depends on the state (whether directly or indirectly) is immediately notified of the change (the 'push'), but derived values are not re-evaluated until they are actually read (the 'pull').
+
+If the new value of a derived is referentially identical to its previous value, downstream updates will be skipped. In other words, Svelte will only update the text inside the button when `large` changes, not when `count` changes, even though `large` depends on `count`:
+
+```svelte
+<script>
+	let count = $state(0);
+	let large = $derived(count > 10);
+</script>
+
+<button onclick={() => count++}>
+	{large}
+</button>
+```