Document that a CupertinoApp isn't necessary for an iOS app (#75549)

packages/flutter/lib/src/cupertino/app.dart

@@ -37,6 +37,11 @@ import 'theme.dart';
 /// This widget also configures the observer of the top-level [Navigator] (if
 /// any) to perform [Hero] animations.
 ///
+/// The [CupertinoApp] widget isn't a required ancestor for other Cupertino
+/// widgets, but many Cupertino widgets could depend on the [CupertinoTheme]
+/// widget, which the [CupertinoApp] composes. If you use Material widgets, a
+/// [MaterialApp] also creates the needed dependencies for Cupertino widgets.
+///
 /// Use this widget with caution on Android since it may produce behaviors
 /// Android users are not expecting such as:
 ///