Improvements for Zuul documentation

[mbuechse]

While working on my Zuul job, I noticed a few potential improvements for the documenation, such as

• for the encryption command line, replace TENANT by SCS, so the reader doesn't need to investigate the tenant name
• stress that names should be unique, so maybe people might want to use project-specific "namespaces" (via prefixes or suffixes)

I can do this myself, I just wanted the issue as a reminder. But other people can do it, too, of course.