Add description for Object3D.renderOrder

src/core/Object3D.d.ts

@@ -120,6 +120,9 @@ export class Object3D extends EventDispatcher {
    */
   frustumCulled: boolean;
 
+	/**
+	 * Overrides the default rendering order of scene graph objects, from lowest to highest renderOrder. Opaque and transparent objects remain sorted independently though. When this property is set for an instance of Group, all descendants objects will be sorted and rendered together.
+	 */
   renderOrder: number;
 
   /**