update

Author: LadyBluenotes

src/routes/reference/rendering/render-to-string.mdx

@@ -69,8 +69,9 @@ Rendered HTML string.
 
 ## Behavior
 
-- `renderToString` completes synchronously.
-- The output includes hydration data for progressive hydration.
+- `renderToString` is a server rendering API, is unsupported in browser bundles, and completes synchronously.
+- The output includes hydration markup. Inline serialized scripts are emitted only when serializer data is produced.
+- Registered assets are injected into the HTML output, typically before `</head>`.
 - `renderId` namespaces the render output when multiple top-level roots are present.
 
 ## Examples