clarify storage of timestamps and refs in the index
clarify storage of timestamps and refs in the index

keegancsmith/prometheus,prometheus/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,kangwoo/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,alileza/prometheus,prometheus/tsdb,alileza/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,kangwoo/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/tsdb,GoogleCloudPlatform/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus