message: Explain why we keep -Zfeatures=compare
diff: @@ -931,10 +931,13 @@ impl CliUnstable {
  "host-config" => self.host_config = parse_empty(k, v)?,
  "target-applies-to-host" => self.target_applies_to_host = parse_empty(k, v)?,
  "features" => {
  - // For now this is still allowed (there are still some
  - // unstable options like "compare"). This should be removed at
  - // some point, and migrate to a new -Z flag for any future
  - // things.
  + // `-Z features` has been stabilized since 1.51,
  + // but `-Z features=compare` is still allowed for convenience
  + // to validate that the feature resolver resolves features
  + // in the same way as the dependency resolver,
  + // until we feel confident to remove entirely.
  + //
  + // See rust-lang/cargo#11168
  let feats = parse_features(v);
  let stab_is_not_empty = feats.iter().any(|feat| {
  matches!(
  