Doc: Stale info about QML module versioning

https://doc.qt.io/qt-6/qtqml-modules-identifiedmodules.html#semantics-of-identified-modules 
says that for Identified Modules, "clients must specify a version when importing the module". This is no longer true.
 
The final paragraph in that section also takes a lot of effort to explain importing "com.example.CustomUi 1.0". Changing to an unversioned import also lets us avoid making assumptions in the text.