I came across Decorations but from the reference documentation alone it is not clear to me what they should be used for. They seem to be somehow node related, but are declared on the editor view? Is there some introduction article somewhere? Or a small example?
I think CodeMirror has a concept of decorations. There’s a plugin for “annotating” the scrollbar to show markers on it, for instance. Maybe PM is similar? 
No, that’s a different thing. I think the docs do a relatively good job explaining what they do. A proper guide with examples will be written in the future, when 1.0 is out.