Doc update: clarify version of flags.

the navigation bar and the status bar:</p>

<pre>View decorView = getWindow().getDecorView();

// Hide both the navigation bar and the status bar.

// SYSTEM_UI_FLAG_FULLSCREEN is only available on Android 4.1 and higher, but as

// a general rule, you should design your app to hide the status bar whenever you

// hide the navigation bar.

int uiOptions = View.SYSTEM_UI_FLAG_HIDE_NAVIGATION

              | View.SYSTEM_UI_FLAG_FULLSCREEN;

decorView.setSystemUiVisibility(uiOptions);</pre>