docs: add attachToDocument replacement example (vuejs#1578)

Author: alexmiddeleer

Diff for: docs/api/options.md

@@ -323,7 +323,18 @@ wrapper.destroy()
 - default: `false`
 
 ::: warning Deprecation warning
-`attachToDocument` is deprecated and will be removed in future releases. Use [`attachTo`](#attachto) instead.
+`attachToDocument` is deprecated and will be removed in future releases. Use [`attachTo`](#attachto) instead. Example:
+
+```js
+const elem = document.createElement('div')
+if (document.body) {
+  document.body.appendChild(elem)
+}
+wrapper = mount(Component, {
+  attachTo: elem
+})
+```
+
 :::
 
 Like [`attachTo`](#attachto), but automatically creates a new `div` element for you and inserts it into the body.