SystemInformation singleton has few docs

https://doc-snapshots.qt.io/qt6-dev/qml-qtcore-systeminformation.html

It was added in https://codereview.qt-project.org/c/qt/qtdeclarative/+/398116

I guess the idea was since each property is already documented in QSysInfo, it would be redundant. But usually we are not afraid to copy things like that into docs for QML-facing modules, to have suitable QML syntax available for copying and pasting, for one reason. Of course it would be nice if we could transclude snippets of the descriptive text for each property from the docs for the accessors in QSysInfo, but I guess that's probably not a feature in qdoc so far.