Update directives-reference.mdx with optional root margin

Documentation for optional root margin on client visible.
This documents this pr withastro/astro#9363

src/content/docs/en/reference/directives-reference.mdx

@@ -141,6 +141,12 @@ Load and hydrate the component JavaScript once the component has entered the use
 <HeavyImageCarousel client:visible />
 ```
 
+🆕 **New Feature - Optional rootMargin:** This feature allows for earlier component hydration in scenarios like slow internet connections or when components alter the page layout after loading. By setting a rootMargin, hydration is initiated when this specified margin around the component enters the viewport, rather than the component itself.
+```astro
+<HeavyImageCarousel client:visible="200px" />
+```
+Implementing a rootMargin, such as "200px", is beneficial for reducing layout shifts(CLS) and ensuring components are interactively ready sooner, enhancing the stability and responsiveness of the page.
+
 ### `client:media`
 
 - **Priority:** Low