(PECL solr >= 0.9.2)
SolrDocument::offsetSet — Adds a field to the document
$fieldName
   , string $fieldValue
   ) : voidUsed when the object is treated as an array to add a field to the document.
fieldNameThe name of the field.
fieldValueThe value for this field.
   成功时返回 TRUE, 或者在失败时返回 FALSE。