Add more documentation in ListViewDataSource

Summary:
ListViewDataSource's default data extractor can actually expect another data form:

`{ sectionID_1: [ <rowData1>, <rowData2>, ... ], ... }`
Closes https://github.com/facebook/react-native/pull/1285
Github Author: Zhao Han <cx.chenghai+github@gmail.com>

Test Plan: Changed the ListViewExample to make sure all three formats work.

Libraries/CustomComponents/ListView/ListViewDataSource.js

@@ -103,6 +103,10 @@ class ListViewDataSource {
    *
    *    or
    *
+   *      { sectionID_1: [ <rowData1>, <rowData2>, ... ], ... }
+   *
+   *    or
+   *
    *      [ [ <rowData1>, <rowData2>, ... ], ... ]
    *
    * The constructor takes in a params argument that can contain any of the