[beta] [vm/shared/doc] Add a note about experimental nature of NativeCallable.isolateGroupBound method.

This is no-risk documentation change that gives potential users a warning about
experimental nature of NativeCallable.isolateGroupBound method.

CoreLibraryReviewExempt: vm-only change
Cherry-pick: https://dart-review.googlesource.com/c/sdk/+/459501
Change-Id: I36e1380e309a834f37e372bb86a15bc8b8afa656
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460260
Reviewed-by: Nate Biggs <natebiggs@google.com>
Commit-Queue: Alexander Aprelev <aam@google.com>
diff --git a/sdk/lib/ffi/ffi.dart b/sdk/lib/ffi/ffi.dart
index 10ba261..f751c33 100644
--- a/sdk/lib/ffi/ffi.dart
+++ b/sdk/lib/ffi/ffi.dart
@@ -420,6 +420,8 @@
   /// After [NativeCallable.close] is called, invoking
   /// the [nativeFunction] from native code will cause
   /// undefined behavior.
+  ///
+  /// NOTE: This is an experimental feature and may change in the future.
   factory NativeCallable.isolateGroupBound(
     @DartRepresentationOf("T") Function callback, {
     Object? exceptionalReturn,