Improve TabHelper to clarify the use of Namespaces for the nav_link

What does this MR do?

Improve specs to test namespace behavior for the TabHelper#nav_link and ApplicationHelper#current_controller? and improve the method documentation to include examples on how to specify a namespace with the controller.

What are the relevant issue numbers?

Part of https://gitlab.com/gitlab-org/gitlab-ee/issues/7126

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides