Improve description of window configs in 'register-val-describe'

* etc/NEWS: Describe the change.
* lisp/register.el (register-val-describe)
[(window-configuration-p (car val))]: Include the shown
buffers in the return value to make
'register-read-with-preview' more useful.  (Bug#30863)

etc/NEWS

@@ -539,6 +539,10 @@ are obsoleted in GVFS.
 Tramp for some look-alike remote file names.
 
 ---
+** Register
+*** The return value of method 'register-val-describe' includes the
+names of buffers shown by the windows of a window configuration.
+
 ** The options.el library has been removed.
 It was obsolete since Emacs 22.1, replaced by customize.
 

lisp/register.el

@@ -391,7 +391,20 @@ Interactively, reads the register using `register-read-with-preview'."
 (cl-defmethod register-val-describe ((val cons) verbose)
   (cond
    ((window-configuration-p (car val))
-    (princ "a window configuration."))
+    (let* ((stored-window-config (car val))
+           (window-config-frame (window-configuration-frame stored-window-config))
+           (current-frame (selected-frame)))
+      (princ (format "a window configuration: %s."
+                     (if (frame-live-p window-config-frame)
+                         (with-selected-frame window-config-frame
+                           (save-window-excursion
+                             (set-window-configuration stored-window-config)
+                             (concat
+                              (mapconcat (lambda (w) (buffer-name (window-buffer w)))
+                                         (window-list (selected-frame)) ", ")
+                              (unless (eq current-frame window-config-frame)
+                                " in another frame"))))
+                       "dead frame")))))
 
    ((frame-configuration-p (car val))
     (princ "a frame configuration."))