doc: Add fallback explanation to SSRResult change

packages/astro/src/types/public/internal.ts

@@ -217,6 +217,7 @@ export interface SSRResult {
 	// serverIslandDynamicBase allows users to specify that server islands will be served from a separate domain.
 	// This is an advanced option and won't be used in most cases. This should only be used if the static assets and
 	// SSR server are served on separate domains.
+	// If this is not set, it will use `base` instead.
 	serverIslandDynamicBase?: string;
 	styles: Set<SSRElement>;
 	scripts: Set<SSRElement>;