docs(upgrade-guide): mention return-object value matching change

closes #22102

Author: KaelWD

packages/docs/src/pages/en/getting-started/upgrade-guide.md

@@ -223,6 +223,7 @@ app.use(vuetify)
 ### v-select/v-combobox/v-autocomplete
 
 - v-model values not present in `items` will now be rendered instead of being ignored.
+- `return-object` no longer matches primitive values in v-model, the initial value must have the same structure as `items` objects.
 - `cache-items` prop has been removed, caching should be handled externally.
 - `item-text` has been renamed to `item-title`, and now looks up the `title` property on item objects by default. `value` is unchanged.
 - `item-disabled` has been removed, and `disabled`, `header`, `divider`, and `avatar` properties are ignored on item objects.