expr: add comment in syntax_tree.rs

src/uu/expr/src/syntax_tree.rs

@@ -161,7 +161,8 @@ impl AstNode {
         {
             let mut out = Vec::with_capacity(operands.len());
             let mut operands = operands.iter();
-
+            // check the first value before `|`, stop evaluate and return directly if it is true.
+            // push dummy to pass the check of `len() == 2`
             if op_type == "|" {
                 if let Some(value) = operands.next() {
                     let value = value.evaluate()?;