Allow disabling the verbose eldoc truncation message

* doc/emacs/programs.texi (Lisp Doc): Document it.

* lisp/emacs-lisp/eldoc.el (eldoc-display-truncation-message): New
variable (bug#43543).
(eldoc--handle-docs): Use it.

doc/emacs/programs.texi

@@ -1291,6 +1291,12 @@ ways.
 This abnormal hook holds documentation functions.  It acts as a
 collection of backends for ElDoc.  This is what modes should use to
 register their documentation functions with ElDoc.
+
+@vindex eldoc-display-truncation-message
+@item eldoc-display-truncation-message
+If non-@code{nil} (the default), display a verbose message about how
+to view a complete documentation (if it has been truncated in the echo
+area).  If @code{nil}, just mark truncated messages with @samp{...}.
 @end table
 
 @node Hideshow

etc/NEWS

@@ -545,9 +545,16 @@ supplied error message.
 which appends a unique suffix to the Edebug name of the current
 definition.
 
-+++
 ** ElDoc
 
++++
+*** New user option 'eldoc-display-truncation-message'.
+If non-nil (the default), eldoc will display a message saying
+something like "(Documentation truncated. Use `M-x eldoc-doc-buffer'
+to see rest" when a message has been truncated.  If nil, truncated
+messages will be marked with just "..." at the end.
+
++++
 *** New hook 'eldoc-documentation-functions'.
 This hook is intended to be used for registering doc string functions.
 These functions don't need to produce the doc string right away, they
@@ -555,6 +562,7 @@ may arrange for it to be produced asynchronously.  The results of all
 doc string functions are accessible to the user through the user
 option 'eldoc-documentation-strategy'.
 
++++
 *** New user option 'eldoc-documentation-strategy'.
 The built-in choices available for this user option let users compose
 the results of 'eldoc-documentation-functions' in various ways, even

lisp/emacs-lisp/eldoc.el

@@ -67,6 +67,12 @@ If this variable is set to 0, no idle time is required."
 Changing the value requires toggling `eldoc-mode'."
   :type 'boolean)
 
+(defcustom eldoc-display-truncation-message t
+  "If non-nil, provide verbose help when a message has been truncated.
+If nil, truncated messages will just have \"...\" appended."
+  :type 'boolean
+  :version "28.1")
+
 ;;;###autoload
 (defcustom eldoc-minor-mode-string (purecopy " ElDoc")
   "String to display in mode line when ElDoc Mode is enabled; nil for none."
@@ -524,10 +530,13 @@ Honor most of `eldoc-echo-area-use-multiline-p'."
                     (cl-return
                      (concat
                       (buffer-substring (point-min) (point))
-                      (and truncated
+                      (and
+                       truncated
+                       (if eldoc-display-truncation-message
                            (format
                             "\n(Documentation truncated. Use `%s' to see rest)"
-                            (substitute-command-keys "\\[eldoc-doc-buffer]")))))))))
+                            (substitute-command-keys "\\[eldoc-doc-buffer]"))
+                         "..."))))))))
               ((= available 1)
                ;; Truncate "brutally." ; FIXME: use `eldoc-prefer-doc-buffer' too?
                (with-current-buffer (eldoc-doc-buffer)