The text at https://doc.rust-lang.org/error-index.html is stale compared to what appears to be its source material, https://github.com/rust-lang/rust/tree/master/compiler/rustc_error_codes/src/error_codes.
For example, the section for E0038 uses the deprecated dynless syntax Box<Trait>, but the markdown file in Git does not and has been substantially revised.
I found that the file is generated by src/tools/error_index_generator into build/<target>/doc/error-index.html, but I don't know how that file should get deployed to doc.rust-lang.org.