Include documentation on how to implement the QAbstractItemModel::sort function when subclassing

Hi.

The documentation of QAbstractItemModel::sort, as well as the mvp programming guide, indicate that users can (and should) implement the `sort` method when subclassing QAbstractItemModel.

However nowhere does the documentation describe how the function has to be implemented, and no one on the internet seems to know (I've just spent several hours looking). While the standard approach seems to use a QSortFilterProxyModel instead, there are cases in which it's necessary to implement the sort method directly.

Would it be possible to add a description (or at least an example) on how to do so?