Document AssetBundle loadString Decoding Behavior (#88822)

packages/flutter/lib/src/services/asset_bundle.dart

@@ -62,8 +62,12 @@ abstract class AssetBundle {
   /// If the `cache` argument is set to false, then the data will not be
   /// cached, and reading the data may bypass the cache. This is useful if the
   /// caller is going to be doing its own caching. (It might not be cached if
-  /// it's set to true either, that depends on the asset bundle
-  /// implementation.)
+  /// it's set to true either, depending on the asset bundle implementation.)
+  ///
+  /// The function expects the stored string to be UTF-8-encoded as
+  /// [Utf8Codec] will be used for decoding the string. If the string is
+  /// larger than 50 KB, the decoding process is delegated to an
+  /// isolate to avoid jank on the main thread.
   Future<String> loadString(String key, { bool cache = true }) async {
     final ByteData data = await load(key);
     if (data == null)