Document the default value of QSFPM::sortOrder()

...and how to change the value, which is by calling sort().
Also, this is not a property either.

Pick-to: 5.15 6.2 6.3
Change-Id: I1b1be90dc43c29e85903a33d0f99569fd6c87abb
Reviewed-by: David Faure <david.faure@kdab.com>

src/corelib/itemmodels/qsortfilterproxymodel.cpp

@@ -2608,9 +2608,12 @@ int QSortFilterProxyModel::sortColumn() const
 
 /*!
     \since 4.5
-    \brief the order currently used for sorting
+    \return the order currently used for sorting
 
-    This returns the most recently used sort order.
+    This returns the most recently used sort order. The default value is
+    Qt::AscendingOrder.
+
+    \sa sort()
 */
 Qt::SortOrder QSortFilterProxyModel::sortOrder() const
 {