docs: add warning about isVisible()

This change updates the documentation to reflect that `isVisible()`
will only work correctly [when attached to the DOM][1] because of an
upstream [`jsdom` issue][2].

[1]: #2016
[2]: jsdom/jsdom#3502

Author: alecgibson

docs/api/index.md

@@ -1433,6 +1433,10 @@ isVisible(): boolean
 
 **Details:**
 
+::: warning
+`isVisible()` only works correctly if the wrapper is attached to the DOM using [`attachTo`](#attachto)
+:::
+
 ```js
 const Component = {
   template: `<div v-show="false"><span /></div>`