Document the need to qt_extract_metatypes or qt_add_library for external libraries exposed in QML modules

I have two classes: ClassA, a QML module defined with the QML_ELEMENT macro, and ClassB, a Qt object class within a separate DLL library using the Q_OBJECT macro.

When inheritting ClassA from ClassB, a warning is generated: "Warning: ClassA.h: ClassB is used but cannot be found." The warning does not impede the execution of the program.

My build system is CMake and Ninja.

Note that the warning is only issued during a clean build！

See attached demo project.