Update Dartdoc for SceneBuilder#addPlatformView (flutter#6957)

lib/ui/compositing.dart

@@ -274,7 +274,20 @@ class SceneBuilder extends NativeFieldWrapperClass2 {
 
   /// Adds a platform view (e.g an iOS UIView) to the scene.
   ///
-  /// This is work in progress and is not currently supported on any platform.
+  /// Only supported on iOS, this is currently a no-op on other platforms.
+  ///
+  /// On iOS this layer splits the current output surface into two surfaces, one for the scene nodes
+  /// preceding the platform view, and one for the scene nodes following the platform view.
+  ///
+  /// ## Performance impact
+  ///
+  /// Adding an additional surface doubles the amount of graphics memory directly used by Flutter
+  /// for output buffers. Quartz might allocated extra buffers for compositing the Flutter surfaces
+  /// and the platform view.
+  ///
+  /// With a platform view in the scene, Quartz has to composite the two Flutter surfaces and the
+  /// embedded UIView. In addition to that, on iOS versions greater than 9, the Flutter frames are
+  /// synchronized with the UIView frames adding additional performance overhead.
   void addPlatformView(int viewId, { Offset offset: Offset.zero, double width: 0.0, double height: 0.0}) {
     assert(offset != null, 'Offset argument was null');
     _addPlatformView(offset.dx, offset.dy, width, height, viewId);