Matt will undoubtedly need to write the documentation for it to it be really a good interpretation of the software. Just like documentation for other software, it's written by people aware of the code. It is most certainly not written like a tutorial and based on consumers opinions of features or results.
This is really probably why we haven't seen one. They can't just hire someone to write a documentation without it becoming a word by word proof read by Matt anyway to ensure there is no miscommunication of the software, if any user could achieve this level of understanding. Like
@Dune said, he's still learning new stuff. Says all there needs to be said. Even by users I know very familiar with algebra and blue nodes coming up with me ways, better performance in their work, etc.
Without forking over the whole code base for some stranger to dissect I don't see any good documentation coming from anyone by Matt.
It
would probably take years to write the docs by just trial and error of mins and maxes and dos and don'ts of everything in TG without referring to source.