qdoc: Unnecessary \brief descriptions on many pages.

Since 5.4, qdoc outputs the brief contents unnecessarily for many pages at the beginning of the page or of the 'Detailed Description' section. This is bad as those briefs are typically not complete sentences.

• QML Type reference: http://doc-snapshot.qt-project.org/qt5-dev/qml-qtquick-item.html#details
• QML module pages: http://doc-snapshot.qt-project.org/qt5-dev/qtquick-xmllistmodel-qmlmodule.html
• Module pages: http://doc-snapshot.qt-project.org/qt5-dev/qtqml-module.html#details