[7.4 rc fix] [doc] add missing configure Redis to use sockets
Add details from 7.2-to-7.3.md. Replaces https://github.com/gitlabhq/gitlabhq/pull/8047.
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
Add details from 7.2-to-7.3.md. Replaces https://github.com/gitlabhq/gitlabhq/pull/8047.