eldoc: modify `eldoc-documentation-function' using `add-function'

* lisp/emacs-lisp/eldoc.el (eldoc-documentation-function): Describe how
major modes should use `add-function' to alter value of the variable.

* lisp/hexl.el (hexl-mode): Set `eldoc-documentation-function' using
`add-function' so the default value is always used.
* lisp/ielm.el (inferior-emacs-lisp-mode): Set
`eldoc-documentation-function' using `add-function' so the default
value is always used.
* lisp/progmodes/cfengine.el (cfengine3-mode): Set
`eldoc-documentation-function' using `add-function' so the default
value is always used.
* lisp/progmodes/elisp-mode (emacs-lisp-mode): Set
`eldoc-documentation-function' using `add-function' so the default
value is always used.
* lisp/progmodes/octave.el (octave-mode): Set
`eldoc-documentation-function' using `add-function' so the default
value is always used.
* lisp/progmodes/python.el (python-mode): Set
`eldoc-documentation-function' using `add-function' so the default
value is always used.
* lisp/simple.el (read--expression): Set `eldoc-documentation-function'
using `add-function' so the default value is always used.

1 files changed, 2 insertions, 2 deletions

diff --git a/lisp/progmodes/python.el b/lisp/progmodes/python.el
index 1e8623dd901..d298f96bc81 100644
--- a/lisp/progmodes/python.el
+++ b/lisp/progmodes/python.el
@@ -4662,8 +4662,8 @@ Arguments START and END narrow the buffer region to work on."
                                                  (current-column))))
          (^ '(- (1+ (current-indentation))))))
 
-  (set (make-local-variable 'eldoc-documentation-function)
-       #'python-eldoc-function)
+  (add-function :before-until (local 'eldoc-documentation-function)
+                #'python-eldoc-function)
 
   (add-to-list 'hs-special-modes-alist
                `(python-mode "^\\s-*\\(?:def\\|class\\)\\>" nil "#"