add comment about NS_OBJECT_ENSURE_REGISTERED() macro to the manual

doc/manual/attributes.texi

@@ -207,6 +207,8 @@ and some type of global default value.
 In the ns-3 attribute system, these value definitions and accessor
 functions are moved into the TypeId class; e.g.:  
 @verbatim
+NS_OBJECT_ENSURE_REGISTERED (DropTailQueue);
+
 TypeId DropTailQueue::GetTypeId (void) 
 {
   static TypeId tid = TypeId ("ns3::DropTailQueue")
@@ -239,6 +241,11 @@ strings such as "MaxPackets" and TypeId strings.  In the next
 section, we will provide an example script that shows how users
 may manipulate these values.
 
+Note that initialization of the attribute relies on the macro
+NS_OBJECT_ENSURE_REGISTERED (DropTailQueue) being called; if you leave
+this out of your new class implementation, your attributes will not be 
+initialized corretly.
+
 @subsection Basic usage
 
 Let's look at how a user script might access these values.