Added a warning regarding async setup (#2105)

* chore: added a warning regarding async setup

* Update src/api/composition-api-setup.md

Co-authored-by: skirtle <[email protected]>

* fix: removed tips

Co-authored-by: skirtle <[email protected]>

Author: NataliaTepluhina

src/api/composition-api-setup.md

@@ -38,11 +38,11 @@ export default {
 </template>
 ```
 
-Note that [refs](/api/reactivity-core.html#ref) returned from `setup` are [automatically shallow unwrapped](/guide/essentials/reactivity-fundamentals.html#deep-reactivity) when accessed in the template so you do not need to use `.value` when accessing them. They are also unwrapped in the same way when accessed on `this`.
+[refs](/api/reactivity-core.html#ref) returned from `setup` are [automatically shallow unwrapped](/guide/essentials/reactivity-fundamentals.html#deep-reactivity) when accessed in the template so you do not need to use `.value` when accessing them. They are also unwrapped in the same way when accessed on `this`.
 
-:::tip
 `setup()` itself does not have access to the component instance - `this` will have a value of `undefined` inside `setup()`. You can access Composition-API-exposed values from Options API, but not the other way around.
-:::
+
+`setup()` should return an object _synchronously_. The only case when `async setup()` can be used is when the component is a descendant of a [Suspense](../guide/built-ins/suspense.html) component.
 
 ## Accessing Props {#accessing-props}