bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Stefan Monnier]

> Why you don't consider the alternative with less invasive changes, is beyond
> my understanding.

[ It's the same old maintainer's conundrum, trying to balance the need
  to stay in control of the code and make sure it's coherent etc.. while
  at the same time delegating and encouraging participation.  ]

To a large extent, I tend to give a lot of weight to existing code.

I don't have the time or energy to work on eldoc myself, and apparently
neither do you, so the one who writes the code gets to take most of
the decisions.

>From what I remember the last time I looked at the branch, there were
things I didn't much like, but not more so than in the rest of Emacs ;-)

I can't ask everyone to code what I would write (and it's often for the
better since what I'd write would likely not work because I'm not aware
of the underlying difficulties that would make it fail).


        Stefan