Added documentation.

modules/highgui/doc/user_interface.rst

@@ -83,6 +83,9 @@ If window was created with OpenGL support, ``imshow`` also support :ocv:class:`o
 
 .. note:: This function should be followed by ``waitKey`` function which displays the image for specified milliseconds. Otherwise, it won't display the image. For example, ``waitKey(0)`` will display the window infinitely until any keypress (it is suitable for image display). ``waitKey(25)`` will display a frame for 25 ms, after which display will be automatically closed. (If you put it in a loop to read videos, it will display the video frame-by-frame)
 
+.. note::
+
+    [Windows Backend Only] Pressing Ctrl+C will copy the image to the clipboard.
 
 namedWindow
 ---------------