chore(base): Add comments to AST nodes

[jubnzv]

This makes it easier to work with the code base, because sometimes a developer could forget about meaning of some arguments of the expr and stmt constructors.

I use the same commenting style as in the OCaml compiler: https://github.com/ocaml/ocaml/blob/af13e29e0499d479035b064d9d70d9614e76a4fe/parsing/parsetree.mli#L285

[anton-trunov]

This is really useful, thank you! Btw, using anonymous records in constructors would really help remembering the meanings of AST node components and also with standardizing the naming scheme of AST node components. Perhaps this is too big of a refactoring, though.

[jubnzv]

Here is an issue for the further refactoring: #1160