The rule sounds as the following:
Unique features and rules - Common sense = Help Center
If you’re using third-party software to run your community, you’ll likely have all the documentation in place and all you will need to do is to adjust it for your case.
If you are developing your own platform, you will have to write all the documentation yourself. Your task here is to describe the key principles of how your tools work as well as how to use these tools effectively, plus the unique cultural characteristics of your community. In order to keep the help center short, you need to focus only on aspects that cause misunderstanding among a large number of users. The help center should contain only descriptions of the unique features of the platform and community minus what an average person understands without any additional explanation.
Also it is important to remember that when new tools get added to the platform or rules of the community get changed, you will need to update the help center. The less you have in it, the less time you will spend maintaining it.
This is a fragment of a draft of the book “Lessons Learned While Working On Stack Overflow”. Read the full book on kindle or the paperback version.