Fix the preconditoins in the library docs

[akrzemi1]

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.