Update documentation for `PlatformException.stacktrace` (#114028)

* Update message_codec.dart

* remove whitespace

* Update doc comment

* Update message_codec.dart

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

@@ -187,6 +187,10 @@ class PlatformException implements Exception {
   /// ```
   ///
   /// In the cases described above, the content of [stacktrace] will be the unprocessed output of calling `toString()` on the exception.
+  ///
+  /// MacOS, iOS, Linux and Windows don't support querying the native stacktrace.
+  ///
+  /// On custom Flutter embedders this value will be null on platforms that don't support querying the call stack.
   final String? stacktrace;
 
   @override