Make customization example more clear with a heading before each option

example/doc_custom_vector.cpp

@@ -20,6 +20,10 @@ int main ()
 {
    using namespace boost::container;
 
+//--------------------------------------------
+//          'stored_size' option
+//--------------------------------------------
+
    //This option specifies that a vector that will use "unsigned char" as
    //the type to store capacity or size internally.
    typedef vector_options< stored_size<unsigned char> >::type size_option_t;
@@ -43,6 +47,10 @@ int main ()
 
    assert(exception_thrown == true);
 
+//--------------------------------------------
+//          'growth_factor' option
+//--------------------------------------------
+
    //This option specifies that a vector will increase its capacity 50%
    //each time the previous capacity was exhausted.
    typedef vector_options< growth_factor<growth_factor_50> >::type growth_50_option_t;