Qbs documentation not registered (but qbs.qch is shipped)

1. Install Qt 5.9 or a standalone Qt Creator 4.3.0.
2. Start Qt Creator
3. Check the registered documentation at Menu:Tools->Options->Help->Tab:Documentation:
   ==> There is no qbs entry.

...but the necessary *.qch file is shipped:

~/opt/qtcreator-4.3.1_notReleased % find . -name "*qbs*.qch"
./share/doc/qtcreator/qbs.qch