Update docs based on customer upgrade from 4.0

## What does this MR do?

Adds helpful notes to early upgrade docs. Without these helpful hints it will be really hard for most people to upgrade from early versions to later versions. 

The tip about doing database migrations to version 6.1, then migrating iids, then migrating the database all the way was based on a tip from https://github.com/gitlabhq/gitlabhq/issues/7869. It worked really well!


See merge request !4409