Improve doc of `mouse-drag-mode-line-buffer'

* doc/emacs/frames.texi (Mouse Commands): Document
`mouse-drag-mode-line-buffer'.
* etc/NEWS: Explain where that option is supported.
* lisp/mouse.el (mouse-drag-mode-line-buffer): Likewise.

doc/emacs/frames.texi

@@ -136,6 +136,12 @@ entirely on the screen.  The number of lines scrolled per step depends
 on how far away from the window edge the mouse has gone; the variable
 @code{mouse-scroll-min-lines} specifies a minimum step size.
 
+@vindex mouse-drag-mode-line-buffer
+  If you enable the option @code{mouse-drag-mode-line-buffer} and
+dragging files is supported by the window system, then dragging the
+mouse on the buffer name portion of the mode line will drag that
+buffer's file to another program or frame.
+
 @findex mouse-yank-primary
 @findex mouse-yank-at-click
   Clicking with the middle mouse button, @kbd{mouse-2}, moves point to

etc/NEWS

@@ -305,9 +305,11 @@ the current Emacs process at the end, it starts a new Emacs process
 optional parameters to restart instead of just killing the current
 process.
 
++++
 ** New user option 'mouse-drag-mode-line-buffer'.
 If non-nil, dragging on the buffer name part of the mode-line will
-drag the buffer's associated file to other programs.
+drag the buffer's associated file to other programs.  This option is
+currently only available on X, Haiku and Nextstep (GNUstep or macOS).
 
 +++
 ** New user option 'mouse-drag-and-drop-region-cross-program'.

lisp/mouse.el

@@ -111,7 +111,10 @@ down and up respectively."
 (defcustom mouse-drag-mode-line-buffer nil
   "If non-nil, allow dragging files from the mode line.
 When the buffer has an associated file, it can be dragged from
-the buffer name portion of its mode line to other programs."
+the buffer name portion of its mode line to other programs.
+
+This option is only supported on X, Haiku and Nextstep (GNUstep
+or macOS)."
   :type 'boolean
   :version "29.1")