diffguard-core: bare URLs in doc comments — clippy::bare_url_links

[EffortlessSteven]

Two doc comment modules have bare URLs that trigger :\n\n1. \n \n\n2. \n \n\nFix: Wrap bare URLs in so rustdoc renders them as proper links:\n\n\n\nPriority: Low — cosmetic doc fix, but concrete and quick to fix.