minor #15877 Document by_reference option (Herz3h)

This PR was submitted for the 5.3 branch but it was merged into the 4.4 branch instead.

Discussion
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Document by_reference option

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

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Commits
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4d0c9b6 Document by_reference option

Author: wouterj

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