Document by_reference option

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

Author: Herz3h

reference/forms/types/entity.rst

@@ -37,6 +37,7 @@ objects from the database.
 |             | from the :doc:`FormType </reference/forms/types/form>`:          |
 |             |                                                                  |
 |             | - `attr`_                                                        |
+|             | - `by_reference`_                                                 |
 |             | - `data`_                                                        |
 |             | - `disabled`_                                                    |
 |             | - `empty_data`_                                                  |
@@ -327,6 +328,8 @@ 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