bug#62029: 29.0.60; Allow users to customize eldoc buffer separator

["João Távora" <joaotavora@gmail.com>]

Yuan Fu <casouri@gmail.com> writes:

> I agree. But in the same time, eldoc’s rendering should be
> customizable too. If someone uses eldoc-doc-buffer and want to use
> fancier separators, they should be able to. That’ll be easier than
> writing another displayer that does everything the same as
> eldoc-doc-buffer except for using a different separator.

I've pushed two commits to master that address the design shortcoming I
was referring to.  Here they are summarized:

commit e19994fe8c000b0ed2dbc667cdec26cf54356907
    ElDoc: rework rendering of echo area (bug#62029)
    
    Previously, the display function 'eldoc-display-in-echo-area' reused
    the same buffer as 'eldoc-display-in-doc-buffer', but that made it
    harder to render documentation items differently depending on the
    specific constraints of each display functions.

commit 9b18407c7fd91313544acfb3457be5447987e20a
    ElDoc: remember origin backend in doc snippets (bug#62029)
    
    This lays groundwork for discriminating between different
    documentation providers in ElDoc display outlets, i.e. members
    of eldoc-display-functions

The first commit detaches eldoc-display-in-echo-area from
eldoc-display-in-doc-buffer (It also allows the backend to control the
display in the echo area specifically).  For the purposes of this bug,
it should allow eldoc-display-in-doc-buffer to more freely structure the
*eldoc* buffer with separators, etc.

The second commit adds a :origin cookie to each doc item passed to
eldoc-display-functions.  The origin is the name of the backend itself.
It could allow for better separators in the *eldoc* buffer where each
section is titled according to the backend that produced the
documentation.

So I guess what's missing for this bug is a customizable *eldoc* buffer
separator with a good default value, like one of those resize-resisting
horizontal separators.  Also, for customization, maybe `format-spec' can
be used?

João