DOCS/man/options: document special wid value -1

This value can be used to tell mpv to detach from the existing parent
into its own window.

DOCS/man/options.rst

@@ -3711,12 +3711,15 @@ Window
     environments. This functionality was removed in 0.33.0, but it is possible to
     call the ``xdg-screensaver`` command line program from a user script instead.
 
-``--wid=<ID>``
+``--wid=<ID|-1>``
     This tells mpv to attach to an existing window. If a VO is selected that
     supports this option, it will use that window for video output. mpv will
     scale the video to the size of this window, and will add black bars to
     compensate if the aspect ratio of the video is different.
 
+    An ID of value ``-1`` is interpreted specially, and mpv will detach from
+    the currently attached window to its own window.
+
     On X11, the ID is interpreted as a ``Window`` on X11. Unlike
     MPlayer/mplayer2, mpv always creates its own window, and sets the wid
     window as parent. The window will always be resized to cover the parent