docs(ads): add documentation for adThemeStyle in loadNativeAd method

- Clarify the purpose of the adThemeStyle parameter
- Explain that it provides UI-agnostic theme properties for ad styling
- Improve code readability and understanding for developers

Author: fulleni

lib/ads/ad_provider.dart

@@ -27,6 +27,7 @@ abstract class AdProvider {
   ///
   /// The [imageStyle] is used to select an appropriate native ad template
   /// that best matches the visual density of the surrounding content.
+  /// The [adThemeStyle] provides UI-agnostic theme properties for ad styling.
   Future<app_native_ad.NativeAd?> loadNativeAd({
     required HeadlineImageStyle imageStyle,
     required AdThemeStyle adThemeStyle,