bug#41531: 27.0.91; Better handle asynchronous eldoc backends

[Eli Zaretskii <eliz@gnu.org>]

> From: João Távora <joaotavora@gmail.com>
> Date: Tue, 30 Jun 2020 12:31:10 +0100
> Cc: Stefan Monnier <monnier@iro.umontreal.ca>, andreyk.mad@gmail.com,
>  Dmitry Gutov <dgutov@yandex.ru>
> 
> The work that started with this discussion is now mostly complete.  It
> has been sitting in the scratch/eldoc-async branch of the Savannah repo
> for a while, but I've been very busy and didn't have time to annouce it.

Thanks.

> Anyway I think my efforts are ready to push to master.  I'll do so soon
> unless someone raises a serious problem about it.

Not a serious problem, but some comments:

> -(defun eldoc-message (&optional string)
> +(make-obsolete
> + 'eldoc-message "use `eldoc-documentation-functions' instead." "1.1.0")

Isn't the version part of the obsolete message supposed to tell the
version of Emacs?

The change to the number of arguments of the functions in the
eldoc-documentation-functions hook is backward-incompatible, isn't it?
I see you've changed the relevant functions in our sources, but what
about 3rd-party packages?

Also, the doc string of this hook needs clarification regarding the
arguments: it first says that CALLBACK is the only mandatory argument
to the hook functions, but then, out of the blue, appear additional
arguments DOCSTRING and a list of key-value pairs.  Confusing.

The doc strings have some words in UK English spelling "(e.g.,
"honour"), please fix that.  Also, please make sure comments all start
with a capital letter, end with a period, and comprise full English
sentences.

The doc string of eldoc-documentation-compose doesn't say a word about
the function's argument.

In the doc string of eldoc-documentation-strategy, you use the phrase
"queries the special hook for all functions that produce doc strings"
to mean, AFAIU, that the specified functions in the hook-variable list
are called.  IMO, this wording could be confusing; suggest to use this
instead:

  `eldoc-documentation-compose': calls all the functions in the hook,
  and displays all of the resulting doc strings ...

This doc string doesn't explain the use of the timer, it explains the
reason for its existence.  It should also describe the use:

> +(defvar eldoc--enthusiasm-curbing-timer nil
> +  "Timer used by `eldoc-documentation-enthusiast' to avoid blinking.")

Last, but not least: the "async" part of the branch's name hints on
some advanced and extremely useful functionality that these changes
are supposed to allow, but I see no mention of that in NEWS and in the
manual bits.  What did I miss?