Add comments to 3 methods that were missing comments. (#11737)

* Add comments to 3 methods that were missing comments.

Fix out of order (getter, private field, setter) tuple

* Code review fix.

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

@@ -247,28 +247,34 @@ class _WidgetInspectorState extends State<WidgetInspector>
 class InspectorSelection {
   /// Render objects that are candidates to be selected.
   ///
-  /// Tools may wish to iterate through the list of candidates
+  /// Tools may wish to iterate through the list of candidates.
   List<RenderObject> get candidates => _candidates;
-
+  List<RenderObject> _candidates = <RenderObject>[];
   set candidates(List<RenderObject> value) {
     _candidates = value;
     index = 0;
   }
 
-  List<RenderObject> _candidates = <RenderObject>[];
-
   /// Index within the list of candidates that is currently selected.
   int index = 0;
 
+  /// Set the selection to empty.
   void clear() {
     _candidates = <RenderObject>[];
     index = 0;
   }
 
+  /// Selected render object from the [candidates] list.
+  ///
+  /// Setting [candidates] or calling [clear] resets the selection.
+  ///
+  /// Returns null if the selection is invalid.
   RenderObject get current {
     return candidates != null && index < candidates.length ? candidates[index] : null;
   }
 
+  /// Whether the selected render object is attached to the tree or has gone
+  /// out of scope.
   bool get active => current != null && current.attached;
 }