Improve `PlatformException#stacktrace` docs for Android (#104331)

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

@@ -145,6 +145,51 @@ class PlatformException implements Exception {
   ///   print(stacktrace);
   /// }
   /// ```
+  ///
+  /// On Android this field is populated when a `RuntimeException` or a subclass of it is thrown in the method call handler,
+  /// as shown in the following example:
+  ///
+  /// ```kotlin
+  /// import androidx.annotation.NonNull
+  /// import io.flutter.embedding.android.FlutterActivity
+  /// import io.flutter.embedding.engine.FlutterEngine
+  /// import io.flutter.plugin.common.MethodChannel
+  ///
+  /// class MainActivity: FlutterActivity() {
+  ///   private val CHANNEL = "channel_name"
+  ///
+  ///   override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
+  ///     super.configureFlutterEngine(flutterEngine)
+  ///     MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
+  ///       call, result -> throw RuntimeException("Oh no")
+  ///     }
+  ///   }
+  /// }
+  /// ```
+  ///
+  /// It is also populated on Android if the method channel result is not serializable.
+  /// If the result is not serializable, an exception gets thrown during the serialization process.
+  /// This can be seen in the following example:
+  ///
+  /// ```kotlin
+  /// import androidx.annotation.NonNull
+  /// import io.flutter.embedding.android.FlutterActivity
+  /// import io.flutter.embedding.engine.FlutterEngine
+  /// import io.flutter.plugin.common.MethodChannel
+  ///
+  /// class MainActivity: FlutterActivity() {
+  ///   private val CHANNEL = "channel_name"
+  ///
+  ///   override fun configureFlutterEngine(@NonNull flutterEngine: FlutterEngine) {
+  ///     super.configureFlutterEngine(flutterEngine)
+  ///     MethodChannel(flutterEngine.dartExecutor.binaryMessenger, CHANNEL).setMethodCallHandler {
+  ///       call, result -> result.success(Object())
+  ///     }
+  ///   }
+  /// }
+  /// ```
+  ///
+  /// In the cases described above, the content of [stacktrace] will be the unprocessed output of calling `toString()` on the exception.
   final String? stacktrace;
 
   @override