Document tricks for power users vs. bigger downloads for automatic upgrade

Doing a manual upgrade (not including IUKs) will make the IUKs you download for the next release smaller. Power users might want to know about this so they can optimize their bandwidth usage. And they (e.g. if they are an instructor) can tell users with such needs about it.

This is not part of the official deliverables for Sponsor 2 but we thought that it was an important task anyway.