Specs: clean up the last remaining mentions of documents

sky/specs/gestures.md

 Gestures
 ========
 
 ```dart
 SKY MODULE
 <!-- part of dart:sky -->
 
 <script>
 abstract class GestureEvent extends Event {
   Gesture _gesture;
@@ skipping 152 matching lines @@
     return new PointerState(gestures: source.gestures, chosen: source.chosen);
   }
   @nonnull List<@nonnull Gesture> gestures;
   @nonnull bool chosen = false;
 }
 
 class GestureManager {
   GestureManager(this.target) {
     target.events.where((event) => event is PointerDownEvent).listen(_handler);
   }
-  final @nonnull EventTarget target; // usually the ApplicationDocument object
+  final @nonnull EventTarget target; // usually the ApplicationRoot object
 
   Map<@nonnull int, @nonnull PointerState> _pointers = new SplayTreeMap<int, PointerState>();
 
   void addGesture(@nonnull PointerEvent event, @nonnull Gesture gesture) {
     assert(gesture.active);
     var pointer = event.pointer;
     if (_pointers.containsKey(pointer)) {
       assert(!_pointers[pointer].gestures.contains(gesture));
       if (_pointers[pointer].chosen)
         cancelGesture(gesture);
@@ skipping 237 matching lines @@
   // fling-end: cancel(), or after fling or fling-cancel (prechoose)
 }
 
 class FlingLeftGesture : FlingGesture { }
 class FlingRightGesture : FlingGesture { }
 class FlingUpGesture : FlingGesture { }
 class FlingDownGesture : FlingGesture { }
 
 </script>
 ```