The fields are rearranged according to the specified criteria and sort direction
Fields can be sorted by boost values, field names and number of values.
The $order_by parameter must be one of :
* SolrInputDocument::SORT_FIELD_NAME
* SolrInputDocument::SORT_FIELD_BOOST_VALUE
* SolrInputDocument::SORT_FIELD_VALUE_COUNT
The sort direction can be one of :
* SolrInputDocument::SORT_DEFAULT
* SolrInputDocument::SORT_ASC
* SolrInputDocument::SORT_DESC