Merge #4083

4083: Smol documentation for ast nodes r=matklad a=Veetaha

There is a tremendous amount of TODOs to clarify the topics I am not certain about.
Please @matklad, @edwin0cheng review carefully, I even left some mentions of your names in todos to put your attention where you most probably can give comments.

In order to simplify the review, I separated the codegen (i.e. changes in `ast/generated/nodes.rs`) from `ast_src` changes (they in fact just duplicate one another) into two commits.

Also, I had to hack a little bit to let the docs be generated as doc comments and not as doc attributes because it's easier to read them this way and IIRC we don't support hints for `#[doc = ""]` attributes for now...

Closes #3682 

Co-authored-by: veetaha <veetaha2@gmail.com>

Author: bors[bot]