message: [TF:XLA] Clarify comment on XlaDeviceContext::TransferLiteralToDevice.
diff: @@ -108,6 +108,11 @@ Status XlaDeviceContext::TransferLiteralToDevice(const Tensor& host_tensor,
  }
  // Unref the host tensor, and capture the literal shared_ptr too so it goes
  // out of scope when the lambda completes.
  + // We don't defer the call to done() onto the stream here, and the reasons why
  + // this is correct are subtle. We assume that:
  + // a) all consumers of the device tensor will wait for its definition event.
  + // b) if the tensor is destroyed, then the memory allocator will not hand out
  + // the same buffers until the transfer has completed.
  host_to_device_stream_->ThenDoHostCallback([ref, literal]() { ref.Unref(); });
  return Status::OK();
  