docs(ui): update Avatar changeset for compound API

Author: alexcarpenter

.changeset/mosaic-avatar.md

@@ -2,4 +2,4 @@
 '@clerk/ui': minor
 ---
 
-Add a new Mosaic `Avatar` component (StyleX) with `shape` (`circle` | `square`) and `size` (`lg` | `md` | `sm` | `xs`) props. Renders an image when `src` is set, otherwise its children as a fallback.
+Add a new Mosaic `Avatar` compound component (StyleX). `Avatar` owns `shape` (`circle` | `square`) and `size` (`lg` | `md` | `sm` | `xs`); compose `AvatarImage` (renders once the image loads) and `AvatarFallback` (shown while the image is pending or has failed, with an optional `delayMs`) inside it.