clarify directive updates, fixes vuejs#1044

Author: chrisvfritz

src/v2/guide/custom-directive.md

@@ -61,9 +61,9 @@ A directive definition object can provide several hook functions (all optional):
 
 - `inserted`: called when the bound element has been inserted into its parent node (this only guarantees parent node presence, not necessarily in-document).
 
-- `update`: called after the containing component has updated, __but possibly before its children have updated__. The directive's value may or may not have changed, but you can skip unnecessary updates by comparing the binding's current and old values (see below on hook arguments).
+- `update`: called after the containing component's VNode has updated, __but possibly before its children have updated__. The directive's value may or may not have changed, but you can skip unnecessary updates by comparing the binding's current and old values (see below on hook arguments).
 
-- `componentUpdated`: called after the containing component __and its children__ have updated.
+- `componentUpdated`: called after the containing component's VNode __and the VNodes of its children__ have updated.
 
 - `unbind`: called only once, when the directive is unbound from the element.