Remove APPEND argument from add-display-text-property

* doc/lispref/display.texi (Display Property): Update doc.
* lisp/emacs-lisp/subr-x.el (add-display-text-property): Remove
the append argument -- it's nonsensical.

doc/lispref/display.texi

@@ -4904,7 +4904,7 @@ with @code{get-char-property}, for instance (@pxref{Examining
 Properties}).
 @end defun
 
-@defun add-display-text-property start end prop value &optional append object
+@defun add-display-text-property start end prop value &optional object
 Add @code{display} property @var{prop} of @var{value} to the text from
 @var{start} to @var{end}.
 
@@ -4922,9 +4922,6 @@ After doing this, the region from 2 to 4 will have the @code{raise}
 the region from 8 to 12 will only have the @code{raise} @code{display}
 property.
 
-If @var{append} is non-@code{nil}, append to the list of display
-properties; otherwise prepend.
-
 If @var{object} is non-@code{nil}, it should be a string or a buffer.
 If @code{nil}, this defaults to the current buffer.
 @end defun

lisp/emacs-lisp/subr-x.el

@@ -471,14 +471,11 @@ This takes into account combining characters and grapheme clusters."
 
 ;;;###autoload
 (defun add-display-text-property (start end prop value
-                                        &optional append object)
+                                        &optional object)
   "Add display property PROP with VALUE to the text from START to END.
 If any text in the region has a non-nil `display' property, those
 properties are retained.
 
-If APPEND is non-nil, append to the list of display properties;
-otherwise prepend.
-
 If OBJECT is non-nil, it should be a string or a buffer.  If nil,
 this defaults to the current buffer."
   (let ((sub-start start)
@@ -504,10 +501,10 @@ this defaults to the current buffer."
                   (list disp))
                  (t
                   disp)))
-          (setq disp
-                (if append
-                    (append disp (list (list prop value)))
-                  (append (list (list prop value)) disp)))
+          ;; Remove any old instances.
+          (when-let ((old (assoc prop disp)))
+            (setq disp (delete old disp)))
+          (setq disp (cons (list prop value) disp))
           (when vector
             (setq disp (seq-into disp 'vector)))
           ;; Finally update the range.