message: Add comments to clarify sparse+ urls
diff: @@ -681,10 +681,13 @@ impl<'a> fmt::Display for SourceIdAsUrl<'a> {
  ref url,
  ..
  } => {
  + // For sparse http registry the URL already contains the prefix.
  if url.scheme().starts_with("sparse+") {
  - write!(f, "{}", url)
  + // e.g. sparse+http://example.com
  + write!(f, "{url}")
  } else {
  - write!(f, "registry+{}", url)
  + // e.g. registry+http://example.com
  + write!(f, "registry+{url}")
  }
  }
  SourceIdInner {
  