(display-buffer): New `pop-up-frames` action alist entry

(display-buffer): New `pop-up-frames` action alist entry

Allow overriding the `pop-up-frames` variable from `display-buffer-alist`
so as to provide a worthy replacement for the old `same-frame`
parameter of `special-display-*` (bug#67249).

* lisp/window.el (special-display-popup-frame): Declare obsolete;
that was apparently forgotten back when `special-display-*` variables
were declared obsolete.
Use the new `pop-up-frames` action alist entry instead of the variable.
(display-buffer): Document new alist entry.
(window--pop-up-frames): New function.
(display-buffer--maybe-pop-up-frame, display-buffer-in-previous-window)
(display-buffer-reuse-window, display-buffer-reuse-mode-window): Use it.

* doc/lispref/windows.texi (Choosing Window Options): Mention that
`pop-up-frames` is also an action alist entry.
(Buffer Display Action Alists): Add `pop-up-frames` entry.