Thank you for your interest in contributing to the Rust Reference!
There are a few ways of helping with the reference: critiquing the reference, editing the reference, fixing incorrect information, adding examples and glossary entries, and documenting new or otherwise undocumented features in Rust.
For awhile, the Reference was basically ignored, and Rust continued gaining new features or changing old ones. It was also basically the introduction document before the first edition of the Rust book, and constantly in flux from the huge churn of the language design before v1.0.0. So there's a lot that's wrong, too teachy for people who should have basic understanding of Rust, or is too shallow for the Reference. As such, we have the warning saying there's work that needs to be done. Eventually, we plan to make sure everything is well documented enough that we can remove the warning.
This is the easiest way to contribute. Basically, as you read the reference, if you find something confusing, incorrect, or missing, then you can file an issue against the reference explaining your concerns.
Typos and incorrect links get through from time to time. Should you find them, we welcome PRs to fix them. Additionally, larger editing jobs that help remove the number of parentheticals, remove comma splices, italicize term definitions and other similar tasks are helpful.
Examples are great. Many people will only read examples and ignore the prose. Ideally, every facet of every feature will have an example.
Likewise, the reference has a glossary. It doesn't need to explain every facet of every feature nor contain every definition, but it does need to be expanded upon. Ideally entries in the glossary link to the associated documentation.
There are a lot of features that are not documented at all or are documented poorly. This is the hardest, but definitely most valuable. Pick something from "Document all features" or a missing feature tag, and write about it.
While writing, you may find it handy to have a playpen open to test out what you are documenting.
Feel free to take information from the standard library and Rustonomicon as appropriate.
Note that we don't write documentation for purely library features such as threads and IO and we don't write about Rust in the future. Documentation is written as if the current stable release of Rust is the last release. If you want to write about Rust in the future, you want the Unstable book.
Older, stable RFCs need review to determine if they need documentation written and, if so, need that documentation written. The "Document all features" issue tracks the overall effort, and individual RFCs are laid out on the RFC Status project. RFCs that have not yet been reviewed to scope out the work are in the "Needs Review" column, with ones needing documentation in the "Stable, Needs Documentation" column.
If you review an RFC and determine that there is no documentation required, please convert the project card to an issue and then close the issue, explaining why no documentation is required. This is so as to ensure that there is a record and a chance for others to disagree. If you review it and determine documentation is necessary, feel free to simply move the card into the "Stable, Needs Documentation" column. It can be converted into an issue if it needs discussion, or left as a text card.
For RFCs which do not stabilize all at once (for instance, because some aspects are insta-stable), if there is any unstabilized part that needs documentation, then the RFC should be advanced to "Stable, Needs Documentation" and converted to an issue. On the issue, remark about which parts need documentation and which are still unstable.
Now, in order for a new RFC to be stabilized, it must have documentation written. If this requires a change to the reference, then the necessary documentation should be written and a PR created. Once the PR has been reviewed (along with any necessary documentation PRs to other repositories), the feature can be stabilized in Rust, and then the doc PRs merged. Anyone is free to write these PRs, but they should wait until the feature is unlikely to change much before stabilization.
RFCs needed documentation for stabilization can be added to the RFC Status project, under the "Awaiting Docs for Stabilization" column.