Update stale Cutoff comments: the failing pair reads it too (#166)

backend/internal/store/admin.go

@@ -30,13 +30,14 @@ const (
 
 // SeriesFilter is one named filter predicate over the whole library. Site
 // and Kind narrow the row read; Name picks the predicate; Cutoff is the
-// staleness boundary the "stale" filter compares against, supplied by the
+// staleness boundary the age-based filters — "stale" and the failing pair —
-// caller's clock — the store has no clock; Page is 1-based.
+// compare against, supplied by the caller's clock — the store has no clock;
+// Page is 1-based.
 type SeriesFilter struct {
 	Site   string // "" = every Site
 	Kind   string // "" = both library buckets' series
 	Name   string // one of the SeriesFilter* constants; "" = SeriesFilterAll
-	Cutoff int64  // unix ms; "stale" reads it, the store never does
+	Cutoff int64  // unix ms; the age-based filters read it, the store never does
 	Page   int    // 1-based page of the row read; default 1
 }