Update docs

docs/platforms/flutter/index.mdx

@@ -70,12 +70,14 @@ Future<void> main() async {
 }
 ```
 
-```dart {tabTitle:Flutter Web}  {"onboardingOptions": {"performance": "13-15", "profiling": "16-19"}}
+```dart {tabTitle:With custom zone}  {"onboardingOptions": {"performance": "16-18", "profiling": "19-22"}}
 import 'package:flutter/widgets.dart';
 import 'package:sentry_flutter/sentry_flutter.dart';
 
 Future<void> main() async {
-  // By using Sentry.runZonedGuarded you can have a custom Zone, where you can call `WidgetsBinding` before `SentryFlutter.init`, with automatic error and breadcrumb reporting.
+  // The SDK creates it's own custom zone on web for automatic error and breadcrumb tracking on web.
+  // This could lead to zone mismatch errors if you needed to call `WidgetsBinding.ensureInitialized()` before Sentry in a cusom zone.
+  // With `Sentry.runZonedGuarded` you still get convenient auto error and breadcrumb tracking and can also call `WidgetsBinding.ensureInitialized()` before Sentry.
   Sentry.runZonedGuarded(() async {
     WidgetsBinding.ensureInitialized();