Document what is the final resource path created by qt_add_qml_module

Context:

https://git.qt.io/alcroito/qt6-cmake-api-review/-/commit/1129de590fd595b97f162114745224a712d08956#179fb5213a114272e340c63285f9fb35ba530ab8_490_553

Document that the final resource path of is a concatenation of ${RESOURCE_PREFIX} + ${TARGET_PATH}.
We don't have a better idea than '/' as the default, so we keep it to make things smoother.