docs: use function bindings in "when not to use effect" (#15544)

Author: Rich-Harris

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

@@ -280,62 +280,34 @@ You might be tempted to do something convoluted with effects to link one value t
 </label>
 ```
 
-Instead, use callbacks where possible ([demo](/playground/untitled#H4sIAAAAAAAACo1SMW6EMBD8imWluFMSIEUaDiKlvy5lSOHjlhOSMRZeTiDkv8deMEEJRcqdmZ1ZjzzxqpZgePo5cRw18JQA_sSVaPz0rnVk7iDRYxdhYA8vW4Wg0NnwzJRdrfGtUAVKQIYtCsly9pIkp4AZ7cQOezAoEA7JcWUkVBuCdol0dNWrEutWsV5fHfnhPQ5wZJMnCwyejxCh6G6A0V3IHk4zu_jOxzzPBxBld83PTr7xXrb3rUNw8PbiYJ3FP22oTIoLSComq5XuXTeu8LzgnVA3KDgj13wiQ8taRaJ82rzXskYM-URRlsXktejjgNLoo9e4fyf70_8EnwncySX1GuunX6kGRwnzR_BgaPNaGy3FmLJKwrCUeBM6ZUn0Cs2mOlp3vwthQJ5i14P9st9vZqQlsQIAAA==)):
+Instead, use `oninput` callbacks or — better still — [function bindings](bind#Function-bindings) where possible ([demo](/playground/untitled#H4sIAAAAAAAAE51SsW6DMBT8FcvqABINdOhCIFKXTt06lg4GHpElYyz8iECIf69tcIIipo6-u3f3fPZMJWuBpvRzkBXyTpKSy5rLq6YRbbgATdOfmeKkrMgCBt9GPpQ66RsItFjJNBzhVScRJBobmumq5wovhSxQABLskAmSk7ckOXtMKyM22ItGhhAk4Z0R0OwIN-tIQzd-90HVhvy2HsGNiQFCMltBgd7XoecV2xzXNV7XaEcth7ZfRv7kujnsTX2Qd7USb5rFjwZkJlgJwpWRcakG04cpOS9oz-QVCuoeInXW-RyEJL-sG0b7Wy6kZWM-u7CFxM5tdrIl9qg72vB74H-y7T2iXROHyVb0CLanp1yNk4D1A1jQ91hzrQSbUtIIGLcir0ylJDm9Q7urz42bX4UwIk2xH2D5Xf4A7SeMcMQCAAA=)):
 
 ```svelte
 <script>
 	let total = 100;
 	let spent = $state(0);
 	let left = $state(total);
 
-	function updateSpent(e) {
-		spent = +e.target.value;
+	function updateSpent(value) {
+		spent = value;
 		left = total - spent;
 	}
 
-	function updateLeft(e) {
-		left = +e.target.value;
+	function updateLeft(value) {
+		left = value;
 		spent = total - left;
 	}
 </script>
 
 <label>
-	<input type="range" value={spent} oninput={updateSpent} max={total} />
+	<input type="range" bind:value={() => spent, updateSpent} max={total} />
 	{spent}/{total} spent
 </label>
 
 <label>
-	<input type="range" value={left} oninput={updateLeft} max={total} />
+	<input type="range" bind:value={() => left, updateLeft} max={total} />
 	{left}/{total} left
 </label>
 ```
 
-If you need to use bindings, for whatever reason (for example when you want some kind of "writable `$derived`"), consider using getters and setters to synchronise state ([demo](/playground/untitled#H4sIAAAAAAAACpWRwW6DMBBEf8WyekikFOihFwcq9TvqHkyyQUjGsfCCQMj_XnvBNKpy6Qn2DTOD1wu_tRocF18Lx9kCFwT4iRvVxenT2syNoDGyWjl4xi93g2AwxPDSXfrW4oc0EjUgwzsqzSr2VhTnxJwNHwf24lAhHIpjVDZNwy1KS5wlNoGMSg9wOCYksQccerMlv65p51X0p_Xpdt_4YEy9yTkmV3z4MJT579-bUqsaNB2kbI0dwlnCgirJe2UakJzVrbkKaqkWivasU1O1ULxnOVk3JU-Uxti0p_-vKO4no_enbQ_yXhnZn0aHs4b1jiJMK7q2zmo1C3bTMG3LaZQVrMjeoSPgaUtkDxePMCEX2Ie6b_8D4WyJJEwCAAA=)):
-
-```svelte
-<script>
-	let total = 100;
-	let spent = $state(0);
-
-	let left = {
-		get value() {
-			return total - spent;
-		},
-		set value(v) {
-			spent = total - v;
-		}
-	};
-</script>
-
-<label>
-	<input type="range" bind:value={spent} max={total} />
-	{spent}/{total} spent
-</label>
-
-<label>
-	<input type="range" bind:value={left.value} max={total} />
-	{left.value}/{total} left
-</label>
-```
-
 If you absolutely have to update `$state` within an effect and run into an infinite loop because you read and write to the same `$state`, use [untrack](svelte#untrack).