Hide implementation comment we leaked into docs

Also, add some dartdocs for AppLifecycleState values.

sky/engine/core/dart/window.dart

@@ -17,11 +17,16 @@ typedef void PointerPacketCallback(ByteData serializedPacket);
 typedef void AppLifecycleStateCallback(AppLifecycleState state);
 
 /// States that an application can be in.
-///
-/// These values must match the order of the values of AppLifecycleState
-/// in sky_engine.mojom
 enum AppLifecycleState {
+  // These values must match the order of the values of
+  // AppLifecycleState in sky_engine.mojom
+
+  /// The application is not currently visible to the user. When the
+  /// application is in this state, the engine will not call the
+  /// [onBeginFrame] callback.
   paused,
+
+  /// The application is visible to the user.
   resumed,
 }