bug#69662: 30.0.50; PGTK Window system supports drag-and-drop but doesn't define x-begin-drag

[Eli Zaretskii]

> From: Po Lu <luangruo@yahoo.com>
> Cc: bjorn.bidar@thaodan.de,  69662@debbugs.gnu.org
> Date: Fri, 15 Mar 2024 21:24:57 +0800
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Which doc strings mention that?
> 
> dired-mouse-drag-files mentions that:
> 
> This feature is supported only on X Windows, Haiku, and Nextstep (macOS
> or GNUstep).
> 
> and mouse-drag-mode-line-buffer:
> 
> This option is only supported on X, Haiku and Nextstep (GNUstep or
> macOS).
> 
> for example.  (elisp)Drag and Drop also features this disclaimer:
> 
>    It is also possible to drag content from Emacs to other programs when
>    this is supported by the current window-system.

Thanks, I added the same caveat to a few more functions' doc strings.