Clarify the doc index

With regards to the unstable book, the reference, and the
processes involved.

Also, fix up a link by pointing to the new tracking issue rather than
the older one.

Fixes #41285

src/doc/index.md

@@ -32,16 +32,21 @@ nicknamed 'The Rust Bookshelf.'
 * [The Rustonomicon][nomicon] is your guidebook to the dark arts of unsafe Rust.
 * [The Reference][ref] is not a formal spec, but is more detailed and comprehensive than the book.
 
+Initially, documentation lands in the Unstable Book, and then, as part of the
+stabilization process, is moved into the Book, Nomicon, or Reference.
+
 Another few words about the reference: it is guaranteed to be accurate, but not
-complete. We now have a policy that all new features must be included in the
-reference before stabilization; however, we are still back-filling things that
-landed before then. That work is being tracked [here][38643].
+complete. We have a policy that features must have documentation to be stabilized,
+but we did not always have this policy, and so there are some stable things that
+are not yet in the reference. We're working on back-filling things that landed
+before this policy was put into place. That work is being tracked
+[here][refchecklist].
 
 [Rust Learning]: https://github.com/ctjhoa/rust-learning
 [Docs.rs]: https://docs.rs/
 [api]: std/index.html
 [ref]: reference/index.html
-[38643]: https://github.com/rust-lang/rust/issues/38643
+[refchecklist]: https://github.com/rust-lang-nursery/reference/issues/9
 [err]: error-index.html
 [book]: book/index.html
 [nomicon]: nomicon/index.html