moved option "targetDocument" to optional attributes

As documented at http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/embedded-mapping.html#mixing-document-types and http://docs.doctrine-project.org/projects/doctrine-mongodb-odm/en/latest/reference/reference-mapping.html#mixing-document-types you can omit the targetDocument option if you want to store different types of documents in references or embedded documents -> targetDocument is an optional option of @ReferenceMany and @EmbedMany