We really need to add some grouping support to that thing. I would like to
propose the following strategy:

1) make --help not output anything about all the registered defaultvalues.
Output help only for the user-registered values.

2) introduce a set of macros:
DEFAULT_VALUE_GROUP_DEFINE ("Wifi");
DEFAULT_VALUE_GROUP_ADD ("Wifi", "WifiRtsCtsThreshold");
DEFAULT_VALUE_GROUP_ADD ("Wifi", "WifiFragmentationThreshold");
...

this would create a --help-Wifi option which would be displayed by default and
the --help-Wifi option would further display all the options registered in the
'Wifi' group.

I have not given much thinking to the implementation and I am sure that other
APIs could be used. But I suspect that this would go a long way towards making
the CommandLine thing useful. It would also be nice to use it to classify the
options in the introspected output I have implemented in the ns-3-wifi branch.