Add documentation

Signed-off-by: Marcel Müller <neikos@neikos.email>

src/parser/mod.rs

@@ -261,6 +261,7 @@ pub enum TemplateAstExpr<'input> {
     ElseConditional {
         expression: Option<Box<TemplateAstExpr<'input>>>,
     },
+    #[expect(unused)]
     Invalid(&'input [TemplateToken]),
     MathOperation {
         op: TokenOperator,