I agree, your suggestion would be an easier way to access the information. In the meantime, I wanted to let you know that the information is very well described, but involves an additional step. What I did was go to this link and type in the terms you're interested in (ie "available water", "allowed depletion", etc.)
The articles are really well written there, IMHO. After the search, make sure that you're looking at the 'Knowledge Base' tab to see the write-ups.
As an example, to address your question of what setting changes do to duration and frequency, check out the table mid-way through this knowledge base article: http://support.rachio.com/article/385-flex-schedule-tips
As a bit of a side note, realize that you don't need to get deep into this stuff to use the Rachio. Per your comment of "I just want a working irrigation system", you might not want to be using Flex Daily. There are other scheduling methods that are not as sensitive to detailed settings, but they won't be as optimal either. What you put in, and how much you get back, is totally up to you. The software gives you the options. Realize, however, that Flex Daily does require some investment of time to understand the settings, and your vegetation and irrigation system in general. In my case, it has paid off incredibly well. I'm going on about 1 year of use with my Rachio now. Last month was my lowest water bill ever, at $8 and some change. I think I watered my lawn once in the entire month, thanks to some rain and cooler temperatures, and I didn't water my trees or shrubs at all in January. I spent a lot of time dialing in settings last spring and early summer, but haven't really touched them since other than when I overseeded my lawn.
Welcome to the community!