docs(analytics): clarify logInAppPurchase API documentation for iOS usage (#18087)

Author: SelaseKay

packages/firebase_analytics/firebase_analytics/lib/src/firebase_analytics.dart

@@ -1209,9 +1209,11 @@ class FirebaseAnalytics extends FirebasePluginPlatform {
   ///
   /// This event signifies that an item(s) was purchased by a user.
   ///
-  /// This API supports manually logging in-app purchase events on iOS.
-  /// This is especially useful in cases where purchases happen outside the native
-  /// billing systems (e.g. custom payment flows).
+  /// This API is intended for manually logging in-app purchase events on iOS,
+  /// particularly for purchases that occur in WebView or non-native billing flows.
+  /// For StoreKit 2 transactions, use [logTransaction] instead.
+  ///
+  /// Only available on iOS.
   Future<void> logInAppPurchase({
     String? currency,
     bool? freeTrial,