Maps/nav overview (C++) documentation is incomplete

Positioning overview (C++) incomplete with lots of things missing.
Please check location-maps-cpp.html.

The overview pages between places and maps/nav seem quite different in the way they're organised: Places includes lots more code snippets and examples, while the Maps/nav overview simply links to other things and explains how they fit together broadly. We should come up with a common structure and massage all of them to fit together.