docs: Clarified when you can put the fragments in the root view.

Per bug b/10641907, putting fragments in the root view *only* works
if the device runs Android 3.0 or later. Chris and Adam say
we don't really want to suggest that work flow even for these devices,
so I'm taking that info out.

bug: 10641907
Change-Id: Ie44acbb534b8e0cea8dbbfaaf2c65836b1c31be6