doc: rewritten decorator introduction

doc/examples/decorator_02.cpp

@@ -0,0 +1,26 @@
+//  (C) Copyright Andrzej Krzemienski 2015.
+//  Distributed under the Boost Software License, Version 1.0.
+//  (See accompanying file LICENSE_1_0.txt or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+
+//  See http://www.boost.org/libs/test for the library home page.
+
+//[example_code
+#define BOOST_TEST_MODULE decorator_02
+#include <boost/test/included/unit_test.hpp>
+namespace utf = boost::unit_test;
+
+BOOST_AUTO_TEST_SUITE(suite1, *utf::label("trivial"))
+
+  BOOST_AUTO_TEST_CASE(test_case1)
+  {
+    BOOST_TEST(true);
+  }
+
+  BOOST_AUTO_TEST_CASE(test_case2)
+  {
+    BOOST_TEST(1 == 1);
+  }
+
+BOOST_AUTO_TEST_SUITE_END()
+//]