The generated documentation for QtDBus is lacking the Doxygen ( ?) tags file (a file which allows linking e.g. from other QCH files into the QCH file for QtDbus).
Seems that http://code.qt.io/cgit/qt/qtbase.git/tree/src/dbus/doc/qtdbus.qdocconf is missing a line
tagfile = ../../../doc/qtdbus/qtdbus.tags
which seems to have been lost on commit http://code.qt.io/cgit/qt/qtbase.git/commit/src/dbus/doc/qtdbus.qdocconf?id=f810c9fc330e41230321442e3fa06edb2fd95ee6
(Issue found when using https://phabricator.kde.org/D2854 with KDE Frameworks modules which link to QtDBus, so their generated QCH file should link to QtDBus class dox in the QtDbus QCH file)