Poor documentation for QSortFilterProxyModel::sortRole and filterRole

The QSortFilterProxyModel::sortRole and filterRole properties are strange. They only affect the default implementations of `filterAcceptsRows` and `lessThan`, virtual methods that you typically reimplement to provide custom filtering or sorting. But when reimplementing, these properties lose their entire value it seems.

More than once, we have had developers who think these properties affect which roles trigger filtering or sorting. You don't want all changes to affect sorting, or filtering, only a few ones.

At best, the docs could be improved to indicate that these properties only work if you don't reimplement the above-mentioned methods...