Document that cross module references do not work with old project structure

https://doc.qt.io/qtdesignstudio/studio-designer-developer-workflow.html.

We need a not acout cross references assets between modules.
This is discouraged and only works with the new project strucuture introduced by QDS 4.5.
the old project strucure does not support this for the compiled cmake project.

Add another note like this:

Note:

In case you use the project structure from Qt Design Studio 4.4 and earlier with different explicit import paths like 'imports' or 'asset_imports' referencing assets like images or mesh files inbetween different modules from different import paths will not work in CMake based C++ application we generate.
Such cross module references are not considered good practice, but will work with the new project structure that the wizards propose since Qt Design Studio 4.5.