Slightly improve comments in SliverMultiBoxAdaptorElement (#10939)

I got a bit confused by these comments when trying to debug this code,
let's try clearing them up a little.

packages/flutter/lib/src/widgets/sliver.dart

@@ -577,13 +577,20 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
       performRebuild();
   }
 
-  final SplayTreeMap<int, Element> _childElements = new SplayTreeMap<int, Element>();
+  // We inflate widgets at two different times:
+  //  1. When we ourselves are told to rebuild (see performRebuild).
+  //  2. When our render object needs a new child (see createChild).
+  // In both cases, we cache the results of calling into our delegate to get the widget,
+  // so that if we do case 2 later, we don't call the builder again.
+  // Any time we do case 1, though, we reset the cache.
+
   final Map<int, Widget> _childWidgets = new HashMap<int, Widget>();
+  final SplayTreeMap<int, Element> _childElements = new SplayTreeMap<int, Element>();
   RenderBox _currentBeforeChild;
 
   @override
   void performRebuild() {
-    _childWidgets.clear();
+    _childWidgets.clear(); // Reset the cache, as described above.
     super.performRebuild();
     _currentBeforeChild = null;
     assert(_currentlyUpdatingChildIndex == null);
@@ -596,9 +603,6 @@ class SliverMultiBoxAdaptorElement extends RenderObjectElement implements Render
       } else if (_didUnderflow) {
         lastIndex += 1;
       }
-      // We won't call the delegate's build function multiple times, because we
-      // cache the delegate's results until the next time we need to rebuild the
-      // whole widget.
       for (int index = firstIndex; index <= lastIndex; ++index) {
         _currentlyUpdatingChildIndex = index;
         final Element newChild = updateChild(_childElements[index], _build(index), index);