update frames tracking docs

Author: buenaflor

docs/platforms/flutter/integrations/slow-and-frozen-frames-instrumentation.mdx

@@ -12,9 +12,15 @@ Unresponsive UI and animation hitches annoy users and degrade the user experienc
 This integration can help. Set it up and identify these problems in your app by tracking and showing **slow frames**, **frozen frames**, and **frame delay** metrics for spans.
 Learn more about frame delay [here](https://develop.sentry.dev/sdk/performance/frames-delay/).
 
+<Note>
+
+Frames tracking instrumentation is available on **iOS**, **macOS** and **Android**.
+
+</Note>
+
 ## Instrumentation Behaviour
 
-Frame metrics are manually calculated using the [addPersistentFrameCallback](https://api.flutter.dev/flutter/scheduler/SchedulerBinding/addPersistentFrameCallback.html) API from the Flutter SDK.
+Frame metrics are manually calculated by implementing a custom `WidgetsFlutterBinding` which allows us to measure the duration between [handleBeginFrame](https://api.flutter.dev/flutter/scheduler/SchedulerBinding/handleBeginFrame.html) and [handleDrawFrame](https://api.flutter.dev/flutter/scheduler/SchedulerBinding/handleDrawFrame.html).
 Frame duration tracking in Sentry's Flutter SDK begins automatically when a span starts. It continuously measures each frame's render time until the span finishes, then calculates and attaches frame metrics to the completed span.
 
 ## Prerequisite
@@ -26,22 +32,34 @@ Before starting, ensure:
 
 ## Configure
 
-This type of instrumentation is automatically enabled. There is no need for further configuration.
+This instrumentation is automatically enabled through two integrations:
+- The `WidgetsFlutterBindingIntegration` which provides the custom binding for frame tracking
+- The `FramesTrackingIntegration` which processes and attaches the frame metrics to spans
+
+In most cases, no additional configuration is required.
+
+### Early Widgets Binding Initialization
+
+If you need to initialize the widgets binding earlier than `SentryFlutter.init()`, you must call `SentryWidgetsFlutterBinding.ensureInitialized()` manually. Note that using a different custom binding will prevent this instrumentation from working properly.
+
+Example:
+```dart {2}
+void main() {
+  SentryWidgetsFlutterBinding.ensureInitialized();
+  // ... rest of your initialization code
+}
+```
 
 ## Additional Configuration
 
 ### Disabling the Instrumentation
 
 Set `enableFramesTracking` to `false` in the options to disable the instrumentation.
 
-```dart
-import 'package:flutter/widgets.dart';
-import 'package:sentry_flutter/sentry_flutter.dart';
-
-Future<void> main() async {
-  await SentryFlutter.init(
-    (options) => options.enableFramesTracking = false,
-    appRunner: () => runApp(MyApp()),
-  );
-}
+```dart {3}
+await SentryFlutter.init(
+  (options) {
+    options.enableFramesTracking = false
+  },
+);
 ```