update documentation to include axis information

site/content/docs/04-run-time.md

@@ -768,6 +768,7 @@ Slides an element in and out.
 * `delay` (`number`, default 0) — milliseconds before starting
 * `duration` (`number`, default 400) — milliseconds the transition lasts
 * `easing` (`function`, default `cubicOut`) — an [easing function](/docs#run-time-svelte-easing)
+* `axis` (`x` | `y`, default `y`) — the axis of motion along which the transition occurs
 
 ```sv
 <script>
@@ -776,8 +777,8 @@ Slides an element in and out.
 </script>
 
 {#if condition}
-	<div transition:slide="{{delay: 250, duration: 300, easing: quintOut }}">
-		slides in and out
+	<div transition:slide="{{delay: 250, duration: 300, easing: quintOut, axis: 'x'}}">
+		slides in and out horizontally
 	</div>
 {/if}
 ```