bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Stefan Monnier]

> It is, yes.  The code is clear, not transcendental at all.  But writing
> docstrings is hard.  In fact, it's possibly the hardest part of the
> game.  Tell you what: you write the docstring for
> eldoc-documentation-functions, I'll do the implementation.
>
> Deal?

Deal, at the condition that the code comes before I write the docstrings ;-)
[ I like this deal: it's always better for someone else to write the
  docstrings, so at least 2 persons need to agree on what they think the
  code does (assuming the author of the code checks the resulting docstrings).  
]

> I believe it has such functionality that I mentioned before.  And then
> some.  I don't understand it filly but it seems nicely coded, has two
> authors that have (99% sure) assigned copyright.  The library calls
> futures "promises", by the way.

I'm willing to tweak my vocabulary.


        Stefan