The wiki can certainly be improved - however, it isn't intended to document existing standard technologies like XML or SVG - equally, there is excellent documentation available on texturing (GIMP), but also on OpenVG. In other words, while Canvas may be a home-grown thing, there are many concepts it shares with the HTML5 Canvas (wikipedia), e.g. the way styling works.
However, in FlightGear, everything is using an actual scene graph, which is hardware accelerated using OSG/OpenGL.
You are unlikely to make much progress with Canvas if you don't have some previous experience with object-oriented programming and graphics in particular. The only thing the wiki can do, and which is doing already, is introducing high level concepts and FlightGear specifics, while providing more specific pointers to other resources.
The fgdata/base package level interface you need to know to use the Canvas system is fairly lightweight actually, but you will definitely need to be experienced with object-oriented scripting and graphics concepts (transformations, translations, rotations, transparency, rgba, textures).
Like Thorsten said, the Canvas system itself really only provides a single top-level group (think FOLDER) per Canvas, which can contain other child elements, including raster images, text, OpenVG paths, maps or other groups - i.e. it's a fully recursive thing, groups can have groups can have groups. So you need to understand how the tree data structure works, and how it relates to the FlightGear property tree.
The power behind the Canvas system in FlightGear is certainly not due to its documentation, but because it's using industry standards and concepts that people can easily read up on - without things having to be specific to FlightGear/SimGear.
If you don't have a clue about these things, tinkering around with the Canvas Snippets article, the Nasal Console and a few tutorials may help you learn more.
Anyway, don't expect any of us here to actually spoonfeed all this info to you (or others) - there simply is not enough time to do that (no offense), so the only thing that seems to work is refining existing docs/tutorials based on feedback people provide - as far as I can tell, you have not actually responded with any such feedback yet. However, I am more than willing to review such feedback and help improve the docs accordingly, but I cannot introduce basic concepts like OOP, FlightGear scripting, closures, listeners or timers etc - you gotta do some of your own legwork first, and demonstrate that you have actually read up on the corresponding topics - even if that just means looking at stuff that is unrelated to FlightGear.
Otherwise, these topics really look astoundingly similar to wannabe aircraft developers struggling with the XML format, and PropertyList encoded XML in particular, as well as the notion of what a property is/becomes in FlightGear, and how its types relate to the property tree and the way XML data is serialized back and forth between the property tree.
You will agree that, as long as people don't understand these concepts, it makes zero sense to explain much more difficult concepts.
PS: I find your SVG related questions redundant, because "SVG" actually does have a corresponding entry which answers your question, and contains more pointers:
http://wiki.flightgear.org/SVGAnyway, Canvas Snippets is probably what you want to take a look at (wiki), and probably learn more about Inkscape along the way - there are snippets and a MFD framework that you can use without having to know much about coding, just by creating your artwork in Inkscape and using a snippet of Nasal code to load it.