I think you would want to use the "Execute Query" Server Behavior and not the Update Server Behavior.
The Update server behavior is for updating the database based on php code (usually form elements). In your example you appear to be updating one row based on another row's value. That can be done entirely in sql using the "Execute Query" server behavior which supports the Concat() function (that is a mySQL function and not a php function).
You can also add a where clause to the end of the SQL statement like:
WHERE documentation IS NOT NULL AND documentation <> ''