Got universal strings to notify for missing translations.

src/document.rs

@@ -10,6 +10,6 @@ use record::{InternalRecord, InternalRecords, Oid};
 pub use clock::Clock;
 pub use create::{CreateDoc, IndexType};
 pub use definition::{DocDef, DocFuncType};
-pub use field::{Field, FieldType};
+pub use field::{Field, FieldType, MissingTranslation};
 pub use record::{Record, Records};
 pub use session::Session;