bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Stefan Monnier]

> The current discussion, and the current eldoc-async branch adds some more
> design decisions, as well as breaking changes.

I don't see any breaking changes (other than to things which haven't
yet been released IMO).

> As soon as we get futures/promises/aio into the core, that will cease to be
> the case.

Then let's get moving on that.  No need to wait.

> But I fear we wouldn't be able to roll back the related decisions
> so easily, however.

I don't see any reason to fear that.  The more time we spend discussing
what the ideal should look like, the less time we have to actually
get there.

The current eldoc-async branch doesn't get us further from the ideal,
I believe, unless `emacs-28` gets cut before we get our act together.

But if we don't get our act together before `emacs-28` then the
alternative is to have Emacs-28 without support for async eldoc, which
I think is even worse.

I recommend we try and be pragmatic.  Especially since it will make us
all happier (instead of arguing against each other, we get to work on
improving the code).


        Stefan