Allow passing a list of modes to `+mapc-some-buffers'

Due to the way this is written, I can't pass one mode by itself---it must be a
list.

lisp/acdw.el

@@ -99,14 +99,20 @@ If body executes without errors, MESSAGE...Done will be displayed."
 By default, act on all buffers.
 
 Both PREDICATE and FUNC are called with no arguments, but within
-a `with-current-buffer' form on the currently-active buffer."
+a `with-current-buffer' form on the currently-active buffer.
+
+As a special case, if PREDICATE is a list, it will be interpreted
+as a list of major modes.  In this case, FUNC will only be called
+on buffers derived from one of the modes in PREDICATE."
   (let ((pred (or predicate t)))
     (dolist (buf (buffer-list))
       (with-current-buffer buf
-        (when (if (or (eq (car-safe pred) 'closure)
-                      (fboundp pred))
-                  (funcall pred)
-                pred)
+        (when (cond ((or (eq (car-safe pred) 'closure)
+                         (fboundp pred))
+                     (funcall pred))
+                    ((listp pred)
+                     (apply #'derived-mode-p pred))
+                    (t pred))
           (funcall func))))))
 
 ;; https://github.com/cstby/emacs.d/blob/main/init.el#L67