parameters should have a description

[dirk-thomas]

As provided for dynamic reconfigure parameters:

• https://github.com/ros/dynamic_reconfigure/blob/4e643b94a2487f6b70b9483e2ea657bc3b6a1030/src/dynamic_reconfigure/parameter_generator_catkin.py#L259
• https://github.com/ros/dynamic_reconfigure/blob/4e643b94a2487f6b70b9483e2ea657bc3b6a1030/src/dynamic_reconfigure/parameter_generator_catkin.py#L273
• https://github.com/ros/dynamic_reconfigure/blob/4e643b94a2487f6b70b9483e2ea657bc3b6a1030/src/dynamic_reconfigure/parameter_generator_catkin.py#L279