synchronized_value documentation example fails to compile

The [documentation](https://github.com/boostorg/thread/blob/develop/doc/synchronized_value.qbk) gives the following example. u1 and u2 appear to be variable names, yet they appear to be undeclared.

```
synchronized_value > q1,q2;
void transferMessage()
{
auto lks = synchronize(u1,u2); // dead-lock free algorithm

if(!std::get<1>(lks)->empty())
{
std::get<2>(lks)->push_back(u1->front());
std::get<1>(lks)->pop_front();
}
}

```