bug#41531: 28.0.50; proper Eldoc async support

[Andrii Kolomoiets <andreyk.mad@gmail.com>]

Sorry for the late reply.

João Távora <joaotavora@gmail.com> writes:

>> 1. Display only first line of the hover info.  Again :-)
>
> You should be able to do this with either
>
>    (setq eldoc-echo-area-use-multiline-p 1)
>
> or
>
>    (setq eldoc-echo-area-use-multiline-p nil)
>
> Did you try this? If so, what exactly didn't work for you when you
> did?

This way the signature info is truncated.  For the function with many
parameters the info of the last parameters is not visible. 

>> 2. The hover info is sometimes displayed right before the signature info
>> making the echo area to "blink".  I suppose this must be fixed on Eglot
>> side by not requesting both the hover and the signature infos at the
>> same time.
>
> Not something to be fixed in Eglot, definitely, it's not its fault or
> responsibility: it just reports whatever it has.

According to specification, server may send `triggerCharacters` in the
`SignatureHelpOptions`: the characters that trigger signature help
automatically.  Maybe Eglot should not always request the signature
info.
Though I like the current implementation.

>> 3. That IMO useless "...truncated, see *help* buffer" message is moved
>> to Eldoc.  Do we really need to show this message every time?
>
> I see.  Maybe not _every time_ but at least _once_, I'd say.  Once per
> Eldoc session (but what is an Eldoc session)?  Once per x truncated
> messages?  Customization variable? (I hate those, but maybe).
>
> Or maybe never show it?

Yep. Just like no additional message like "Press C-h v for the full
documentation" is shown hovering the variable in the `emacs-lisp-mode`.