Add documentation on uniqueness of node, topic, and service names

[allsey87]

Feature request

It seems that the current documentation lacks answers to a couple questions I have had while writing my own RMW implementation. For example: are node names within a namespace unique? How about the service and topic names within those nodes? Can I, for example, have two nodes (with different names) containing services with the same service_name?

So far I have looked at the following docs and I think these questions are unanswered. I think this is really important to preserve the ability to change the underlying RMW/DDS implementation in a system and to expect similar results.

• https://github.com/ros2/rmw/blob/master/rmw/include/rmw/rmw.h
• http://design.ros2.org/articles/topic_and_service_names.html
• https://design.ros2.org/articles/ros_middleware_interface.html

Feature description

I would like the above questions to be answered in the official documentation.

[fujitatomoya]

related to ros2/design#187