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, 9 insertions, 6 deletions

diff --git a/lisp/progmodes/cfengine.el b/lisp/progmodes/cfengine.el
index 18df372f2c6..f25b3cb9e2b 100644
--- a/lisp/progmodes/cfengine.el
+++ b/lisp/progmodes/cfengine.el
@@ -1390,12 +1390,15 @@ to the action header."
                  (when buffer-file-name
                    (shell-quote-argument buffer-file-name)))))
 
-  ;; For emacs < 25.1 where `eldoc-documentation-function' defaults to
-  ;; nil.
-  (or eldoc-documentation-function
-      (setq-local eldoc-documentation-function #'ignore))
-  (add-function :before-until (local 'eldoc-documentation-function)
-                #'cfengine3-documentation-function)
+  (if (boundp 'eldoc-documentation-functions)
+      (add-hook 'eldoc-documentation-functions
+                #'cfengine3-documentation-function nil t)
+    ;; For emacs < 25.1 where `eldoc-documentation-function' defaults
+    ;; to nil.
+    (or eldoc-documentation-function
+        (setq-local eldoc-documentation-function #'ignore))
+    (add-function :before-until (local 'eldoc-documentation-function)
+                  #'cfengine3-documentation-function))
 
   (add-hook 'completion-at-point-functions
             #'cfengine3-completion-function nil t)