Update GCE discovery documentation
Update GCE discovery documentation

(cherry picked from commit d4cb81c)

baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo,baishuo/elasticsearch_v2.1.0-baishuo