Add a note about the Android API level requirement for AndroidView. (#21386)

packages/flutter/lib/src/rendering/platform_view.dart

@@ -38,6 +38,8 @@ enum _PlatformViewState {
 
 /// A render object for an Android view.
 ///
+/// Requires Android API level 20 or greater.
+///
 /// [RenderAndroidView] is responsible for sizing, displaying and passing touch events to an
 /// Android [View](https://developer.android.com/reference/android/view/View).
 ///

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

@@ -13,6 +13,8 @@ import 'framework.dart';
 
 /// Embeds an Android view in the Widget hierarchy.
 ///
+/// Requires Android API level 20 or greater.
+///
 /// Embedding Android views is an expensive operation and should be avoided when a Flutter
 /// equivalent is possible.
 ///