Rollup merge of #124011 - fmease:rustdoc-updated-clean-docs, r=GuillaumeGomez

rustdoc: update the module-level docs of `rustdoc::clean`

Let's update this 11-year-old documentation.
This would've helped me greatly when first starting out.

Please point out if I should add, clarify or correct anything.
I plan on looking through the rustc dev guide later to see if anything can be expanded upon over there, too.

Author: GuillaumeGomez