Make clear that onSlidingComplete is called regardless whether value changes

Summary:
Documentation didn't clearly state when onSlidingComplete callback is called.

No tests necessary… simply changing docs.
Closes https://github.com/facebook/react-native/pull/13282

Differential Revision: D4823866

Pulled By: ericvicenti

fbshipit-source-id: 1c3be6ee4d52b097b7558067edf0388739d3f52e

Libraries/Components/Slider/Slider.js

@@ -126,8 +126,9 @@ var Slider = React.createClass({
     onValueChange: PropTypes.func,
 
     /**
-     * Callback called when the user finishes changing the value (e.g. when
+     * Callback that is called when the user releases the slider,
-     * the slider is released).
+     * regardless if the value has changed. The current value is passed
+     * as an argument to the callback handler.
      */
     onSlidingComplete: PropTypes.func,