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.

Parent Task: #15281

Related issues

Blocks #15289

Original created by @anonym on 15288 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information