bug#56659: 29.0.50; nitpicking: eldoc-doc-buffer should use special-mode

[Zhiwei Chen]

M-x eldoc-doc-buffer displays `eldoc--doc-buffer', the ElDoc
documentation buffer, where

#+begin_src elisp
(local-set-key "q" 'quit-window)
#+end_src

is used to imitate the behavior of special-mode. Further more, using
special-mode for `eldoc-doc-buffer' gives users the opportunity to
modify the keymap, add-hook, ... etc. For myself, an evil-mode user,
it's able to press q in normal state to quit the window since
special-mode has already been evilified. Currently, evil adds an advice
after `eldoc-doc-buffer' [1] as a workaround.

[1]: 
https://github.com/emacs-evil/evil-collection/blob/master/modes/eldoc/evil-collection-eldoc.el#L60-L61=

-- 
Zhiwei Chen