Document tracepoint fields

The design document can explain why the tracepoints exist, but it does not explain all the fields. Since the name of some fields might be confusing (e.g. subscription_handle vs. subscription for rclcpp_subscription_init), it would be useful to document that.

However, I'm not sure where would be the best place for that:

• in the tracepoint definition (tp_call.h)
• in the tracepoint interface function definition (tracetools.h)
• in the design document