Document an interface for offline operation.

Complex image builds can download hundreds of MB of data from the
internet with many separate lookups. It would be nice to allow users
to ask for a fast build where those lookups are entirely avoided,
using locally cached resources (where possible). This new interface
allows users to signal to elements that they wish to operate without
updating cached resources, which will in turn allow us to avoid
checking for stale data at all.

As part of this I've also documented where we cache data, so that
things like the ccache cache dir and image cache files are not a
surprise to users.

Change-Id: I27f5de6ceaa4e9c6390721b7c434fe0908df84f5