Add note about mouse_pos to motion event. Fixes kivy#2026.

Author: matham

kivy/input/motionevent.py

@@ -40,6 +40,8 @@ def on_motion(self, etype, motionevent):
 
     Window.bind(on_motion=on_motion)
 
+You can also listen to changes of the mouse position by watching
+:attr:`~kivy.core.window.WindowBase.mouse_pos`.
 
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