Preserve docs on aliases
diff --git a/syntax/mod.rs b/syntax/mod.rs
index f52a582..8ef3c8b 100644
--- a/syntax/mod.rs
+++ b/syntax/mod.rs
@@ -79,6 +79,7 @@
 }
 
 pub struct TypeAlias {
+    pub doc: Doc,
     pub type_token: Token![type],
     pub ident: Ident,
     pub eq_token: Token![=],