Documentation boost.async

Compiler support

This library is supported since Clang 14, Gcc 10 & MSVC 19.28.

Gcc versions 12.1 and 12.2 appear to have a bug for coroutines with out stack variables as can be seen [here](https://godbolt.org/z/6adGcqP1z) and should be avoided for coroutines.

Clang only added std::pmr support in 16, so older clang versions use boost::contianer::pmr as a drop-in replacement.

Some if not all MSVC versions have a broken coroutine implementation, that this library needs to workaround. This may cause non-deterministic behaviour and overhead.

A coroutine continuation may be done in the awaitable returned from a final_suspend, like this:

// in promise
auto final_suspend() noexcept
{
    struct final_awaitable
    {
      std::coroutine_handle<void> continuation{std::noop_coroutine()}; (1)
      bool await_ready() const noexcept;
      std::coroutine_handle<void> await_suspend(std::coroutine_handle<void> h) noexcept
      {
        auto cc = continuation;
        h.destroy(); (2)
        return cc;
      }

      void await_resume() noexcept {}
    };
    return final_awaitable{my_continuation};
};

1	The continuation
2	Self destroying the coroutine before continuation

The final_suspend doesnt not properly suspend the coroutine on MSVC, so that the h.destroy() will cause double destruction of elements on the coroutine frame. Therefor, msvc will need to post the destruction, to do it out of line. This will cause overhead and make the actual freeing of memory indeterministic.