docs: link to attachToDocument definition (vuejs#1432)

close vuejs#1207

Author: sarahdayan

docs/api/wrapper/README.md

@@ -18,7 +18,7 @@ A `Wrapper` is an object that contains a mounted component or vnode and methods
 
 #### `options.attachedToDocument`
 
-`Boolean` (read-only): True if component is attached to document when rendered.
+`Boolean` (read-only): `true` if component is [attached to document](../options.md) when rendered.
 
 ### `selector`