message: Make `doctest` metadata field `false` if target is not doctestable
diff: @@ -308,7 +308,7 @@ impl ser::Serialize for Target {
  .required_features
  .as_ref()
  .map(|rf| rf.iter().map(|s| &**s).collect()),
  - doctest: self.doctest,
  + doctest: self.doctest && self.doctestable(),
  }
  .serialize(s)
  }
  