One of the major tasks of the Documentation Team is to take care of the documentation which comes with every Ubuntu system, and is available in Yelp, the Gnome help system. This documentation is also available on https://help.ubuntu.com.
In order to contribute to the system documentation, you need to know a bit about the tools and processes the team uses to maintain the documentation. Don't worry if you don't know any of our tools yet. They are very easy to learn and it's possible to make useful contributions before learning how to use all the tools.
The first thing you need to do is familiarize yourself with the docteam environment. This page briefly describes the various things you need to know and provides links for further reading and discovery.
The Documentation Wiki is a sub-project of the DocumentationTeam. It is a community-driven and open area of the Ubuntu documentation website which anyone can edit.
also from here https://help.ubuntu.com/community/WikiGuide
This website is free for all to edit, and contributing is easy. You need an account for the website (see /Registration for details), and it is recommended that you read through this guide before contributing.
When should you contribute? Whenever you see something that could be improved or updated! Also, WikiToDo contains details of things that need doing. If you notice an error and do not want to correct it yourself, you can report it as a bug.
So yes, https://help.ubuntu.com is the official documentation but it is possible to contribute to it also. https://help.ubuntu.com/community and https://wiki.ubuntu.com are the same (if we are talking about the documentation of Ubuntu), they are the community maintained documentation. To see that the latter two are the same:
But https://wiki.ubuntu.com is not just about documentation as Braiam correctly pointed out in his above answer.