We now have the main points of the new branch documented in the wiki: https://github.com/icsharpcode/ILSpy/wiki/newdecompiler
The branch will be merged to master when roughly has the same features as the old decompiler engine (~after step 2 on the roadmap).
Documentation: There's only the blog posts and the source code comments. (OK, there were almost no comments in the old decompiler engine; but the new one is a tiny bit better in this regard). There's also David's old dissertation in the doc/ folder in the repository, but that is of practically no relevance to the current state of the decompiler.
newdecompiler also has the "Show steps" button in the ILAst view, where the decompiler tells you which transforms were applied (and allows debugging each step individually). This is quite helpful for tracking down mis-decompilations.