Fix links to QIODeviceBase::OpenMode flags

In class documentation, QIODevice::Read, QIODevice::Write ... is often not linked anymore:

This is due to Qt 6, where the flags being actually defined not in QIODevice, but QIODeviceBase. Therefore qdoc doesn't acknowledge the links anymore in 'heuristic mode' (so without an explicit \l.

Affected documentation (most likely incomplete):

• https://doc.qt.io/qt-6/qsavefile.html#open
• https://doc.qt.io/qt-6/qfile.html#open
• https://doc.qt.io/qt-6/qbuffer.html#open
• https://doc.qt.io/qt-6/qprocess.html#setStandardOutputFile
• https://doc.qt.io/qt-6/qsettings.html#registerFormat
• https://doc.qt.io/qt-6/qtemporaryfile.html#open
• https://doc.qt.io/qt-6/qt.html#endl

Proposed fixes:

Fix type name to QIODeviceBase:: and use explicit link command (\l).