In order to document nested namespaces, you can either enable EXTRAC_ALL:
EXTRACT_ALL = YES
Or document each namespace taking care to reference the full path to the nested namespace:
/**
* @namespace root_namespace
* @brief the root_namespace namespace
*/
namespace root_namespace
{
/**
* @namespace root_namespace::component_namespace
* @brief the component_namespace namespace
*/
namespace component_namespace
{
}
}
