Why do admonitions for `rustdoc` consist only of a warning block?

In AsciiDoc and GitHub's Markdown, admonitions (a.k.a. callouts or alerts) consist of five types (NOTE, TIP, IMPORTANT, CAUTION, and WARNING). However, rustdoc consist only of a warning block. Why is this? Is this an intentional limitation? Looking at usage examples in the clap crate, I think it's also used for purposes like "NOTE" or "TIP".