6743: Don't insert blank lines between doc attributes r=Veykril a=Veykril
Fixes#6742.
Doc attributes should be concatenated via a single linebreak as written in the [rustdoc book](https://doc.rust-lang.org/nightly/rustdoc/the-doc-attribute.html).
Also changed the loop to use an iterator to get rid of the `docs.trim_end_matches("\n\n").to_owned()` part using `Itertools::intersperse`.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
9a88332452