QAbstractProxyModel: Document setSourceModel a bit more

Especially so that subclasses remember to emit reset.

Change-Id: Iadcae3fc5fe72584465d4134f385ed0a1d77bfcd
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/corelib/itemmodels/qabstractproxymodel.cpp

@@ -124,6 +124,10 @@ QAbstractProxyModel::~QAbstractProxyModel()
 
 /*!
     Sets the given \a sourceModel to be processed by the proxy model.
+
+    Subclasses should call beginResetModel() at the beginning of the method,
+    disconnect from the old model, call this method, connect to the new model,
+    and call endResetModel().
 */
 void QAbstractProxyModel::setSourceModel(QAbstractItemModel *sourceModel)
 {