It's easy to understand, they are only two people working on Natron, and they are already struggling with improving the source code. Writing good documentation takes time, and may be done by other people.
In the other hand , each node is well documented if you look into the info button (the one with a ?).
Also there is some user manual, but it's more about very specific things in Natron, like the python API for now.
If you're already used to a compositing application, then understanding how Natron works is quite straightforward.
But for people who want to get into compositing it get a bit harder because you have to translate from Nuke documentation or tutorials to Natron.