Add note to docs about plugins that expect parameters by reference

docs/upgrading.md

@@ -49,6 +49,11 @@ $smarty->registerPlugin('modifier', 'my_special_func', 'my_special_func');
 
 But you may want to consider writing a proper [extension](api/extending/extensions.md).
 
+#### Passing parameters by reference
+You cannot use plugins that expect a parameter by reference anymore. PHP-function such as `reset()`, `prev()`, 
+`next()` and `end()` can be registered as plugin, but they won't work because they expect a parameter by 
+reference and Smarty will try to pass it by value.
+
 ### Removed undocumented tags
 
 Smarty 4 still supported some tags that have been carried over from previous version, but have never been documented.