Update docs.

modules/highgui/doc/user_interface.rst

@@ -218,13 +218,15 @@ Gets the mouse-wheel motion delta, when handling mouse-wheel events EVENT_MOUSEW
 
     :param flags: The mouse callback flags parameter.
 
+For regular mice with a scroll-wheel, delta will be a multiple of 120. The value 120 corresponds to a one notch rotation of the wheel or the threshold for action to be taken and one such action should occur for each delta.
+Some high-precision mice with higher-resolution freely-rotating wheels may generate smaller values.
 
 For EVENT_MOUSEWHEEL positive and negative values mean forward and backward scrolling, respectively.
 For EVENT_MOUSEHWHEEL, where available, positive and negative values mean right and left scrolling, respectively.
 
 .. note::
 
-    Mouse-wheel event are currently available only on Windows. On Windows Units are usually multiples of 120.
+    Mouse-wheel events are currently supported only on Windows.
 
 setTrackbarPos
 ------------------