The Rule Machine documentation is in the process of being updated for Rule 5.1 and re-worked in general, since the current documentation was written upon the release of Rule 4.0 and assumes you know how Rule 3.0 worked (so, IMHO, it's no longer as helpful as it could be for most users). That may address some of your concerns there. I can see where some documentation of specific actions, like "Capture/Restore State" may be helpful, and I also know I've written lots of forum posts where I describe the same. 
(Though that could be an entire document it itself if it describes every single action...)
It is unlikely the Rule 4.x docs will receive much attention at this point given that it is deprecated--it will continue functioning but is no longer available for new installations. Global variables are a Rule 4.x feature that was removed in favor of hub variables in Rule 5.0. Hub variables offer similar features (and more: notably, they are now available to any app that supports them, which includes an increasing number of built-in and some community apps, besides just Rule Machine).
If any app lets you use a variable, it's normally apparent from the UI. In Rule Machine specifically, there are practically unlimited possibilities for what you might want to use a variable for, and in many (most?) rules, you'll perhaps never need one; if you have a specific goal in mind and wonder if a variable might make things easier, consider asking the community for input in a post here. Something in the way of documentation will probably come for the hub variables feature in general at some point, but it's hard to address variables in any exhaustive manner in a "how to" document.
If you notice any other holes in the existing docs or can think of specific docs that may be helpful for new users, I'd suggest adding specific ideas to this thread for awareness!
