Mark, great to hear you're planning on helping write documentation. New contributors are always welcome.
You'll notice from http://docs.moodle.org/20/en/Special:RecentChanges that the wiki is currently undergoing a major clean-up and reorganisation but please don't let that put you off contributing! Simply add documentation of new features wherever you think is appropriate. Alternatively you're welcome to choose a page from http://docs.moodle.org/20/en/Category:Pages_needing_improvement to work on.
Regarding writing guidelines, glad you found MoodleDocs:Guidelines for contributors. However, on my to-do list (somewhere!) is to expand on these guidelines and explain how Moodle Docs is organised these days.
As Tomaz mentioned in his post Update on Moodle 2.0 Docs, previously the teacher and administrator documentation was kept quite separate, whereas from the 2.0 docs onwards, we're aiming to keep all documentation about a particular feature in one place. Thus, for example, when a teacher adds a forum to their course and checks the Forum settings documentation, they can also read about any site administration settings relating to it. Similarly, an administrator can read about course administration settings.
Finally, as Visvanath mentions, the Moodle documentation forum is a great place for discussing these things, so I'm just going to move this thread there...