qdoc: Handle enum classes

"Introduce QAbstractItemModel::checkIndex()" 69496d4e22fb6a8805fbcfa798d9aca53621a2ac https://codereview.qt-project.org/#/c/204377/ introduced the first C++ 11 class enum to Qt (QAIM::CheckIndexOption).

Usage-wise this means its values must be fully qualified (QAIM::CheckIndexOption::IndexIsValid).

The enum values in the \enum doc section were originally qualified accordingly:

\value CheckIndexOption::IndexIsValid 

The qualification was removed by "doc: Fix remaining qdoc warnings in qabstractitemmodel.cpp" 706e8a0295832a54e86fb72e1505444a69a55362 https://codereview.qt-project.org/#/c/216844/ resulting in the doc text:

....
QAbstractItemModel::IndexIsValid 0x0001

The model index passed to QAbstractItemModel::checkIndex() is checked to be a valid model index.
....

which is not correct. The values should show qualified in the documentation