the thing is some people jump straight into the complex stuff without following the simple tutorials first. How would a newbie know about the existence of the $index you mention if he never actually tried to create a simple repeat region to display some dummy data on the page - of course not possible.
How would a newbie find articles like this List of all the dmx functions - #6 by patrick helpful - he won’t. In our experience during the years the more coding we show, the more it scares new users. Then users first impression is “You need to know code to use Wappler” 
It’s a different thing for advanced users, once you’re comfortable with the basics, know how to display dynamic data on the page, using the repeat region options, saw the data picker more than once and noticed $index, then the article could be helpful for you.
So my advice is - start with the basic stuff, you can’t build a rocket without first knowing about the gravity and aerodynamics
And yes, we are actually working on a new docs which will include the methods, properties, actions and options of the extensions included in Wappler: https://docsdev.wappler.io/extensions note that this is not yet done, it’s work in progress.