ast: add use case comment for MatchExpr.var_name

vlib/v/ast/ast.v

@@ -498,7 +498,7 @@ pub:
 	branches      []MatchBranch
 	pos           token.Position
 	is_mut        bool // `match mut ast_node {`
-	var_name      string
+	var_name      string // `match cond as var_name {`
 pub mut:
 	is_expr       bool // returns a value
 	return_type   table.Type