Document by_reference option

This option is needed sometimes, and it is not documented for entityType

Author: Herz3h

reference/forms/types/entity.rst

@@ -284,6 +284,8 @@ type:
 
 .. include:: /reference/forms/types/options/attr.rst.inc
 
+.. include:: /reference/forms/types/options/by_reference.rst.inc
+
 .. include:: /reference/forms/types/options/data.rst.inc
 
 .. include:: /reference/forms/types/options/disabled.rst.inc