docs: Updating permissions preview for Preview 2.

Currently just contains doc additions/clarifications that apply to
both Preview 1 and Preview 2, and also the addition of the new
'storage' bucket and the new shouldShowRequestPermissionRationale()
method.

See first comment for doc stage location.

Change-Id: I0b97741c45e117c5de3fa1a85c9ba0d5e927fa8f