Indicate width and height params are being passed to onContentSizeChange.

Summary:
I had to do a little trial and error to find this out. Would be helpful to have it in the docs.

I'm not sure if there's a standard wording or format you prefer for indicating handler function params.
Closes https://github.com/facebook/react-native/pull/7312

Differential Revision: D3245838

fb-gh-sync-id: 89433c036f7287d9efb69605180734dbc4df4df3
fbshipit-source-id: 89433c036f7287d9efb69605180734dbc4df4df3

Libraries/Components/ScrollView/ScrollView.js

@@ -210,8 +210,11 @@ var ScrollView = React.createClass({
      */
     onScrollAnimationEnd: PropTypes.func,
     /**
-     * Called when scrollable content view of the ScrollView changes. It's
-     * implemented using onLayout handler attached to the content container
+     * Called when scrollable content view of the ScrollView changes.
+     *
+     * Handler function is passed the content width and content height as parameters: `(contentWidth, contentHeight)`
+     *
+     * It's implemented using onLayout handler attached to the content container
      * which this ScrollView renders.
      */
     onContentSizeChange: PropTypes.func,