Add comment about UserObjectQosPolicy

Signed-off-by: Andrea Sorbini <asorbini@rti.com>

rmw_connextdds_common/src/ndds/dds_api_ndds.cpp

@@ -208,6 +208,14 @@ rmw_connextdds_initialize_participant_qos_impl(
 #endif /* RMW_CONNEXT_FAST_ENDPOINT_DISCOVERY */
 
 #if RMW_CONNEXT_SHARE_DDS_ENTITIES_WITH_CPP
+  // UserObjectQosPolicy is an internal, undocumented Connext policy used by the
+  // implementations of different language bindings to control the memory
+  // representations of various objects created by user applications. In this
+  // case, the settings match the requirements of the "modern C++" API, and they
+  // allow the DomainParticipant to be used directly by applications that want
+  // to create new entities in C++11, even though the participant was created
+  // using the C API. If these settings are not specified, an application will
+  // receive a SIGSEGV when trying to create one of these entities.
   dp_qos->user_object.flow_controller_user_object.size = sizeof(void*);
   dp_qos->user_object.topic_user_object.size = sizeof(void*);
   dp_qos->user_object.content_filtered_topic_user_object.size = sizeof(void*);