The interface of `race` (`select`)

The interface of `select` is not clearly documented, so it is difficult to figure out all the available signatures and return types. But judging from the rose in the documentation it is non-uniform.

The non-uniform interface of `select()` (spacial-case for all-void) is problematic and may cause unnecessary difficulties and surprises in generic contexts.

```c++
template // is T a void?
promise f(promise pt, promise pu)
{
auto r = co_await select(pt, pu);
r; // which interface should I use?
// variant or naked size_t?
}
```

Same goes for vector:

```c++
template // is T a void?
promise f(vector<> pv)
{
auto r = co_await select(pv);
r; // which interface should I use?
// pair or naked size_t?
}
```

Different interface calls for a different name. If `promise` is orders of magnitude more frequent than `promise` then offer name `selcetv` for funcitons returning `variant`/`pair`. also, having two different functions would make it easier to document them.

Second issue. Because the documentation is too scarce, I cannot figure out what is returned by the `select` that is given a zero-sized vector of promises.

Third. Why do you use `boost::variant2` rather than `std::variant`? I would expect a rationale in the docs.