Update Appender.AddFast method comment (#551)
Update Appender.AddFast method comment (#551)

Signed-off-by: zhulongcheng <d0a909104c54f7c1064d09716f840641c5a2a6f9@gmail.com>
prometheus/tsdb,keegancsmith/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,kangwoo/prometheus,alileza/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,kangwoo/prometheus,Gouthamve/prometheus,Gouthamve/prometheus,GoogleCloudPlatform/prometheus,kangwoo/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus