FIx docs for list view: "sticky" header not sticky on android

Summary:
:( this confused me, I thought I was missing some configuration or something. But the UIExplorer app has sticky headers on iOS, and non-sticky headers on Android, so I guess that's that.
Closes https://github.com/facebook/react-native/pull/10335

Differential Revision: D4039482

Pulled By: hramos

fbshipit-source-id: 34be2db4b15dec2f28a808a9bbc2eb00b0e35525

Libraries/CustomComponents/ListView/ListView.js

@@ -196,6 +196,8 @@ var ListView = React.createClass({
      * section until it reaches the top of the screen, at which point it will
      * stick to the top until it is pushed off the screen by the next section
      * header.
+     *
+     * NOTE: On Android, the header is not sticky.
      */
     renderSectionHeader: PropTypes.func,
     /**