Add caveats around mixing <script setup> and <script> (#2149)

src/api/sfc-script-setup.md

@@ -239,6 +239,13 @@ export default {
 </script>
 ```
 
+Support for combining `<script setup>` and `<script>` in the same component is limited to the scenarios described above. Specifically:
+
+- Do **NOT** use a separate `<script>` section for options that can already be defined using `<script setup>`, such as `props` and `emits`.
+- Variables created inside `<script setup>` are not added as properties to the component instance, making them inaccessible from the Options API. Mixing APIs in this way is strongly discouraged.
+
+If you find yourself in one of the scenarios that is not supported then you should consider switching to an explicit [`setup()`](/api/composition-api-setup.html) function, instead of using `<script setup>`.
+
 ## Top-level `await` {#top-level-await}
 
 Top-level `await` can be used inside `<script setup>`. The resulting code will be compiled as `async setup()`: