add docs

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

@@ -255,6 +255,30 @@ const destroy = $effect.root(() => {
 destroy();
 ```
 
+## `$effect.active`
+
+The `$effect.active` rune is an advanced feature that indicates whether or not an effect or [async `$derived`](await-expressions) can be created in the current context. To improve performance and memory efficiency, effects and async deriveds can only be created when a root effect is active. Root effects are created during component setup, but they can also be programmatically created via `$effect.root`. 
+
+```svelte
+<script>
+	console.log('in component setup', $effect.active()); // true
+
+	function onclick() {
+		console.log('after component setup', $effect.active()); // false
+	}
+	function ondblclick() {
+		$effect.root(() => {
+			console.log('in root effect', $effect.active()); // true
+			return () => {
+				console.log('in effect teardown', $effect.active()); // false
+			}
+		})();
+	}
+</script>
+<button {onclick}>Click me!</button>
+<button {ondblclick}>Click me twice!</button>
+```
+
 ## When not to use `$effect`
 
 In general, `$effect` is best considered something of an escape hatch — useful for things like analytics and direct DOM manipulation — rather than a tool you should use frequently. In particular, avoid using it to synchronise state. Instead of this...

packages/svelte/src/internal/client/reactivity/effects.js

@@ -48,6 +48,8 @@ import { Batch, schedule_effect } from './batch.js';
 import { flatten } from './async.js';
 
 /**
+ * If an effect can be created in the current context, `VALID_EFFECT_PARENT` is returned. 
+ * If not, a value indicating why is returned. 
  * @returns {number}
  */
 function active_root_effect() {