message: Document concurrency logic in ScatterNdUpdateOp.
diff: @@ -230,6 +230,9 @@ class ScatterNdUpdateOp : public OpKernel {
  const DataType dt_ref = DataTypeToEnum<T>::ref();
  const DataType index_t = DataTypeToEnum<Index>::v();
  dtype_ = c->input_type(0);
  + // If we are updating a resource, we always use the exclusive lock.
  + // For ref types, we lock based on the use_locking parameter
  + // Otherwise, we don't mutate the input tensor (we copy-on-write if needed).
  if (c->input_type(0) == DT_RESOURCE) {
  // TODO(apassos): what to validate here?
  } else if (IsRefType(c->input_type(0))) {
  