Better document how to enable sorting in QSPFM

This can be a big time sink otherwise. Unlike reimplementing
filterAcceptsRow() for filtering, reimplementing lessThan() is
*not* sufficient to enable sorting.
As a drive-by, change \brief to \return because sortOrder() is not
a (formal) property.

Pick-to: 5.15 6.2 6.3
Change-Id: I7e7be6f1e8e6b03a9923a7f77f04005b19a69d41
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/itemmodels/qsortfilterproxymodel.cpp

@@ -2593,9 +2593,12 @@ void QSortFilterProxyModel::sort(int column, Qt::SortOrder order)
 
 /*!
     \since 4.5
-    \brief the column currently used for sorting
+    \return the column currently used for sorting
 
-    This returns the most recently used sort column.
+    This returns the most recently used sort column. The default value is -1,
+    which means that this proxy model does not sort.
+
+    \sa sort()
 */
 int QSortFilterProxyModel::sortColumn() const
 {
@@ -2937,6 +2940,8 @@ void QSortFilterProxyModel::setFilterFixedString(const QString &pattern)
     QComboBox.
 
     The default value is true.
+
+    \sa sortColumn()
 */
 bool QSortFilterProxyModel::dynamicSortFilter() const
 {