Add a comment.

Something that was non-obvious to me.

compiler/rustc_ast/src/ast.rs

@@ -972,7 +972,9 @@ impl UnOp {
     }
 }
 
-/// A statement
+/// A statement. No `attrs` or `tokens` fields because each `StmtKind` variant
+/// contains an AST node with those fields. (Except for `StmtKind::Empty`,
+/// which never has attrs or tokens)
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct Stmt {
     pub id: NodeId,