[docs] document axis parameter

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

@@ -745,6 +745,7 @@ 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'`) — direction of the transition
 
 ```sv
 <script>
@@ -753,7 +754,7 @@ Slides an element in and out.
 </script>
 
 {#if condition}
-	<div transition:slide="{{delay: 250, duration: 300, easing: quintOut }}">
+	<div transition:slide="{{delay: 250, duration: 300, easing: quintOut, axis: 'x' }}">
 		slides in and out
 	</div>
 {/if}