Fix the preconditoins in the library docs

Promise and Generator state that on `get()` exception is thrown if not ready:
https://klemens.dev/async/reference.html#generator-outline
https://klemens.dev/async/reference.html#promise-outline

Are you consciously giving such guarantee? I would expect that to be a precondition on `get()`: call it only if `ready() == true`.

But you should also explain what it means that a result is ready. For instance, when the coroutine threw an exception and it has not been inspected yet by the awaiter, is "result ready"?

The throwing of `std::logic_error` is a poor design choice (also employed in the Standard Library) as it sends a mixed message. Is it a programmer bug when I do that? Or is it something that I can expect deterministic results for?

I would call it a precondition and use `BOOST_ASSERT` in the implementation, and if the user wishes to throw `std::logic_error`, let them install an assertion handler.

For other functions like `async::thread::detach()` the library doesn't specify at all what happens if I call `.detach()` twice. Does `.detach()` have a precondition that `this->joinable() == true`?

Also, what happens when I `co_await` ona `promise` `p` whose `operator bool` returns `false`? Is it a contract violation?

What is the precondition on `co_await p`? is it `p.ready()` or `p. operator bool()`? How are they different?

I should be able to figure out these answers from the documentaiton.