bug#9054: 24.0.50; show source in other window

[Juri Linkov]

>> (defun display-buffer-from-help-p (_buffer-name _action)
>>   (unless current-prefix-arg
>>     (with-current-buffer (window-buffer)
>>       (derived-mode-p 'help-mode))))
>>
>> (push '(display-buffer-from-help-p
>>         display-buffer-same-window
>>         (inhibit-same-window . nil))
>>       display-buffer-alist)
>
> Nice.  It's a bit of a mouthful for users, though.
> ...
> I don't think we can expect users to do all that just to change the
> behaviour of these *Help* commands.  Perhaps if we put the
> `display-buffer-from-help-p' function into Emacs.

Or maybe support specifying a mode directly in the condition part
of display-buffer-alist, but this is still too complex.

> `help-window-select' is (if you squint at it) the kinda-sorta user
> option in the opposite direction, so perhaps the way forward here is to
> just define a new user option, or all a new value (`both'?) that affects
> how windows are selected from the *Help* window.  I guess a new user
> option would be the simplest.

A new user option for buffers outgoing from Help would be fine.