Because K-9 is an open source "group" project the updates are fast and furious and there is no real documentation because hundreds of programmers are contributing to its development. In other words nobody really owns K-9 so it would take one of the contributors to take the time to write the docs. If you check out the K-9 website
k9mail - Project Hosting on Google Code you will learn more about the project. This is also the best place to stay up to date using the Download tab. Download on your Android Device and it will install just like a Market install only faster. Your settings and data will all be in tact.
I am by no means an expert and found it very simple to add any account. K-9 basically does it for you as long as you have your user name, password and incoming and outgoing server information.
The ultimate goal of K-9 is to be incorporated with the stock Android email client and become the default. If that ever happens Google would write the documentation and manufacturers would include it in the device manuals. The app is pretty intuitive and by spending some time going through the settings and experimenting you will quickly learn how to use it and most likely make it your default. Just don't remove any accounts from the clients that are currently working until you have k-9 under control and working the way you want it. This all worked for me and my email experience, which is the most important thing I use my INC for is now terrific, even better than the BB's I came from.