The only semblance of Rust documentation policy/guidelines can be found in src/book/documentation.md, which I think should be moved elsewhere, perhaps to Style guidelines or own file. The chapter can remain in the book, but should only explain how stuff is done, not mandate policy (e.g. "The first line of a documentation comment should be a short summary of its functionality").
One other thing: if we do create documentation policy/guidelines, should it be universal to Rust projects, or just the rust-lang/rust? I suggest we create "Documentation Guidelines", either as a stand-alone document or as a chapter of "Style Guidelines" document, and have contributions to rust-lang/rust documentation follow those guidelines.