Async documented but not functional

[dyfrgi]

Since async() is not implemented in Busted 2, could it be removed from the documentation? https://lunarmodules.github.io/busted/#async-tests shows version 2.1.1 and yet it is not implemented.

[alerque]

It's not that iw was not implemented, it's that it was later removed.

We pulled many of the notes about dependencies and such in #695 and elsewhere, but as you note there are lingering references. PRs removing these irrelevant bits are welcome..