Documentation: Update the Sorting detailed description of QSortFilterProxyModel

Currently, the QSortFilterProxyModel description makes no mention about the sortColumn default value being -1 and the proxy model not sorting due to this default value, nor does it mention that sort() sets the sortColumn to the most recently used sort column. The user may miss this information as it is only available in the sortColumn() function description which is buried deep in the doc.

The suggestion is to update the Detailed Description - Sorting subsection to include this information. Perhaps, the information about the sortColumn beings set to the most recently used column should also be added to the sort() function description. Lastly, a "See also sortColum()" link should be added to the sort() function description.