Document syntax tree enums idiom
diff --git a/src/ty.rs b/src/ty.rs
index 9538260..39bae38 100644
--- a/src/ty.rs
+++ b/src/ty.rs
@@ -16,6 +16,12 @@
 
 ast_enum_of_structs! {
     /// The possible types that a Rust value could have.
+    ///
+    /// # Syntax tree enum
+    ///
+    /// This type is a [syntax tree enum].
+    ///
+    /// [syntax tree enum]: enum.Expr.html#syntax-tree-enums
     pub enum Type {
         /// A dynamically sized slice type: `[T]`.
         pub Slice(TypeSlice {