Rewrite the multisite completely to make sure a new user can understand and use the multisite feature from scratch.
Work started at https://documentation.magnolia-cms.com/display/DOCS/How+to+use+Multisite
=> Instead of just showing config table and the meaning of its properties - explain step by step!

Input from Philip Mundt should go into the new page mentioned above - see https://documentation.magnolia-cms.com/display/DEVINT/Multisite+best+practises+and+common+mistakes

The introduction of the https://documentation.magnolia-cms.com/display/DOCS/Multisite+module needs some rewriting too - after the "How to use Multisite" is done properly.