Expose ElDoc functions in a hook (Bug#28257)

* lisp/emacs-lisp/eldoc.el: Update commentary.
(eldoc--eval-expression-setup): Use new hook.
(eldoc--supported-p): Accomodate new hook.
(eldoc-documentation-functions): New hook.
(eldoc-documentation-default, eldoc-documentation-compose): New
functions.
(eldoc-documentation-function): Use 'eldoc-documentation-default' as new
default value.  Update documentation and custom attributes.
(eldoc-print-current-symbol-info): Accomodate possible null value for
'eldoc-documentation-function'.
* etc/NEWS: Mention them.
* doc/emacs/programs.texi (Emacs Lisp Documentation Lookup): Mention
new hook and changes to 'eldoc-documentation-function'.
* lisp/hexl.el (hexl-mode, hexl-revert-buffer-function):
* lisp/ielm.el (inferior-emacs-lisp-mode):
* lisp/progmodes/cfengine.el (cfengine3-mode):
* lisp/progmodes/elisp-mode.el (emacs-lisp-mode):
* lisp/progmodes/octave.el (octave-mode):
* lisp/progmodes/python.el (python-mode): Use new hook.

1 files changed, 4 insertions, 2 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index a2d85d0bef8..67383b34154 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -5544,8 +5544,10 @@ REPORT-FN is Flymake's callback function."
       ;; Emacs<25
       (set (make-local-variable 'eldoc-documentation-function)
            #'python-eldoc-function)
-    (add-function :before-until (local 'eldoc-documentation-function)
-                  #'python-eldoc-function))
+    (if (boundp 'eldoc-documentation-functions)
+        (add-hook 'eldoc-documentation-functions #'python-eldoc-function nil t)
+      (add-function :before-until (local 'eldoc-documentation-function)
+                    #'python-eldoc-function)))
 
   (add-to-list
    'hs-special-modes-alist