node.c: add more comment to NODE_CASE2

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61271 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

node.c

@@ -218,7 +218,7 @@ dump_node(VALUE buf, VALUE indent, int comment, NODE *node)
 	F_NODE(nd_body, "when clauses");
 	return;
       case NODE_CASE2:
-	ANN("case statement");
+	ANN("case statement with no head");
 	ANN("format: case; [nd_body]; end");
 	ANN("example: case; when 1; foo; when 2; bar; else baz; end");
 	F_NODE(nd_head, "case expr");