Auto merge of #80295 - GuillaumeGomez:beautify-rework, r=petrochenkov

Rework beautify_doc_string so that it returns a Symbol instead of a String

This commit comes from https://github.com/rust-lang/rust/pull/80261, the goal here is to inspect the impact on performance of this change on its own.

The idea of rewriting `beautify_doc_string` is to not go through `String` if we don't need to update the doc comment to be able to keep the original `Symbol` and also to have better performance.

r? `@jyn514`

diff --cc src/librustdoc/clean/types.rs
Simple merge