Document API changes

Author: m-kuhn

doc/api_break.dox

@@ -1066,6 +1066,8 @@ version instead.
 - acceptVisitor() has been removed
 - QgsExpression::referencedColumns() returns QSet<QString> instead of QStringList
 - QgsExpression::Node::referencedColumns() returns QSet<QString> instead of QStringList
+- `QgsExpression::Node` was renamed to `QgsExpressionNode`
+- `QgsExpression::Function` was renamed to `QgsExpressionFunction`
 
 
 QgsExpression::Function       {#qgis_api_break_3_0_QgsExpression_Function}