Document qt_standard_project_setup() better for usage with qml modules

Clarify that qt_standard_project_setup() must come before all qt_add_qml_module() calls, and in the same subdirectory as the executable, otherwise it will cause issues on Windows and MacOS when creating qml modules (with backing libraries) in subdirectories.
qt_standard_project_setup ensures the backing library dlls are placed in the current binary dir next to the executable, so that the windows runtime linker can resolve the dlls.