docs: align setPushNotificationToken iOS mapping with the Swift symbol

The dartdoc referenced setPushNotificationsDeviceTokenString, which does not exist in Swift: NS_SWIFT_NAME renames that ObjC selector to setPushNotificationsDeviceToken(_:), the symbol the plugin actually calls. Documents the real Swift method (String overload) and notes the ObjC origin. Addresses Copilot review comment 3328481126.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: DennisAlund

lib/facebook_app_events.dart

@@ -675,8 +675,11 @@ class FacebookAppEvents {
   /// push-driven app opens and measure push campaigns.
   ///
   /// Platform mapping:
-  /// - iOS: `setPushNotificationsDeviceTokenString`.
-  /// - Android: `setPushNotificationsRegistrationId`.
+  /// - iOS: `AppEvents.setPushNotificationsDeviceToken(_:)` (the String overload).
+  ///   The underlying `setPushNotificationsDeviceTokenString:` Objective-C
+  ///   selector is renamed to `setPushNotificationsDeviceToken(_:)` in Swift via
+  ///   `NS_SWIFT_NAME`, so that is the symbol this plugin calls.
+  /// - Android: `AppEventsLogger.setPushNotificationsRegistrationId`.
   Future<void> setPushNotificationToken(String token) {
     return _channel.invokeMethod<void>('setPushNotificationToken', token);
   }