message: expand comment about incomplete support for interior mutability
diff: @@ -264,11 +264,12 @@ impl<'a, 'mir, 'tcx> EvalContextExt<'tcx> for super::MiriEvalContext<'a, 'mir, '
  borrow_kind: Option<mir::BorrowKind>,
  ) -> EvalResult<'tcx, Borrow> {
  let time = self.machine.stacked_borrows.increment_clock();
  - // FIXME This does not do enough checking when only part of the data has
  - // interior mutability.
  let new_bor = match borrow_kind {
  Some(mir::BorrowKind::Mut { .. }) => Borrow::Mut(Mut::Uniq(time)),
  Some(_) =>
  + // FIXME This does not do enough checking when only part of the data has
  + // interior mutability. When the type is `(i32, Cell<i32>)`, we want the
  + // first field to be frozen but not the second.
  if self.type_is_freeze(pointee_ty) {
  Borrow::Frz(time)
  } else {
  