Document ExternType trait
diff --git a/src/lib.rs b/src/lib.rs
index 32d42a0..fc0e682 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -389,7 +389,10 @@
 pub use crate::exception::Exception;
 pub use crate::extern_type::ExternType;
 pub use crate::unique_ptr::UniquePtr;
-pub use cxxbridge_macro::{bridge, type_id};
+pub use cxxbridge_macro::{bridge};
+
+/// For use in impls of the `ExternType` trait. See [`ExternType`].
+pub use cxxbridge_macro::type_id;
 
 // Not public API.
 #[doc(hidden)]