Document new staleness (#3380)
Document new staleness (#3380)

Remove "interpolation" for this heading, that hasn't
been in these docs for a long time.
brancz/prometheus,Conorbro/prometheus,brancz/prometheus,prometheus/prometheus,AlekSi/prometheus,Stackdriver/stackdriver-prometheus,Stackdriver/stackdriver-prometheus,Stackdriver/stackdriver-prometheus,amorken/prometheus,Conorbro/prometheus,alileza/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,amorken/prometheus,AlekSi/prometheus,kangwoo/prometheus,keegancsmith/prometheus,kangwoo/prometheus,kangwoo/prometheus,prometheus/prometheus,kangwoo/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,brancz/prometheus,amorken/prometheus,Gouthamve/prometheus,alileza/prometheus,Stackdriver/stackdriver-prometheus,AlekSi/prometheus,alileza/prometheus,brancz/prometheus,amorken/prometheus,Conorbro/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,Gouthamve/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,Conorbro/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,Gouthamve/prometheus,AlekSi/prometheus