QtDBus module documentation is missing from non-Linux binary packages

Assuming it's intentional to avoid building this module on Mac and Windows, we are updating the docs to remind users they can build it themselves:

https://codereview.qt-project.org/#change,56614

But it would be nice if this doc could link to the dbus module docs. They aren't being built currently (AFAICT) because the module isn't being built.