Render Eldoc's echo area separately

Render Eldoc's echo area separately

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.

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-functions): Update docstring.
(eldoc--doc-buffer-docs): Remove.
(eldoc--format-doc-buffer): Simplify.
(eldoc--echo-area-render): New helper.
(eldoc-display-in-echo-area): Use 'eldoc--echo-area-render'.

fixup