Docs: Use array shape for perform_test() return type

Author: apermo

src/wp-admin/includes/class-wp-site-health.php

@@ -167,7 +167,17 @@ public function enqueue_scripts() {
 	 * @since 5.4.0
 	 *
 	 * @param callable $callback
-	 * @return mixed
+	 * @return array{
+	 *     label: string,
+	 *     status: 'good'|'recommended'|'critical',
+	 *     badge: array{
+	 *         label: string,
+	 *         color: string,
+	 *     },
+	 *     description: string,
+	 *     actions: string,
+	 *     test: string,
+	 * }
 	 */
 	private function perform_test( $callback ) {
 		/**