This page is an overview of the documentation included with your Rust install. Other unofficial documentation may exist elsewhere; for example, the Rust Learning project collects documentation from the community, and Docs.rs builds documentation for individual Rust packages.
Rust provides a standard library with a number of features; we host its documentation here.
Many of Rust's errors come with error codes, and you can request extended diagnostics from the compiler on those errors. We also have the text of those extended errors on the web, if you prefer to read them that way.
Rust provides a number of book-length sets of documentation, collectively nicknamed 'The Rust Bookshelf.'
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 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.