Add warning about scoped Slot implementation (#1212)

docs/api/options.md

@@ -147,6 +147,27 @@ shallowMount(Component, {
 })
 ```
 
+::: warning Root Element required
+Due to the internal implementatioj of this feature, the slot content has to return a root element, even though a scoped slot is allowed to return an array of elements.
+
+If you ever need this in a test, the recommended workaround is to wrap the component under test in another component and mount that one:
+:::
+
+```javascript
+const WrapperComp = {
+  template: `
+  <ComponentUnderTest v-slot="props">
+    <p>Using the {{props.a}}</p>
+    <p>Using the {{props.a}}</p>
+  </ComponentUnderTest>
+  `,
+  components: {
+    ComponentUnderTest
+  }
+}
+const wrapper = mount(WrapperComp).find(ComponentUnderTest)
+```
+
 ## stubs
 
 - type: `{ [name: string]: Component | boolean } | Array<string>`