Clarify attachedToDocument meaning in docs

Fixes vuejs#1207

Author: dsseng

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 `attachToDocument` in mounting options was `true`
+`Boolean` (read-only): True if component is attached to DOM when rendered.
 
 #### `options.sync`