Comment # 1 on bug 1109367 from Fabian Vogt

Indeed - the packages from the devel prj (both KDE:Qt5 and KDE:Qt:5.11) work
fine.

My first suspicion was qtdoc has a dependency issue and built the documentation
and the module index files in the wrong order, but that turned out to be wrong.

The issue is that qdoc hardcodes the full path to clang system headers, which
breaks if the installed version of clang is different from the clang version
installed during build.
I reported the bug upstream: https://bugreports.qt.io/browse/QTBUG-70687

A workaround will be part of Qt 5.11.2 packages.