Improvement: add clarification hints to Site Wizard webpages

Mentioned in a mailing list discussion: https://lists.debian.org/debian-newmaint/2020/05/msg00030.html

The Site Wizard's "Create guest account" webpage should give a clear estimation of what the applicant gets if application is successful and why the applicant should or shoukd not start the process.

This applies to other Site Wizard pages too.