It's very important to have decent and good documentation. The problem is that often we might have good documenting techniques, but we forget some crucial points. It can be extremely painful to find these crucial points during a disaster recovery.
Therefor I want to create this canonical question and answer on what we should be documenting and how we should be writing it down.
We already have these good questions and answers on what technologies we can use for documenting:
All answers will be merged into one big sectioned answer that administrators can use to checklist their own documentation.