docs(artwork): give each Worker housekeeping method its own godoc

The three methods shared one comment attached to Backfill, so godoc
rendered the other two undocumented and the one it did show described
the group rather than the call. Each now opens with its own name and
says what that call does, including Backfill's bool return.

core/artwork/worker.go

@@ -159,17 +159,20 @@ func (w *Worker) RunPrune(ctx context.Context) error {
 	return prune(ctx, w.proc.ds, w.proc.store)
 }
 
-// Backfill, EnqueueStaleAbsentAll and EnqueueMissingAll are the scheduler's entry points into
-// housekeeping. Like RunPrune they exist so a caller needs only the Worker, not a second
-// DataStore handle onto the state the Worker already owns.
+// Backfill enqueues every entity for re-resolution when the artwork config fingerprint has
+// changed since the last run, artists first. It reports whether anything was enqueued.
 func (w *Worker) Backfill(ctx context.Context) (bool, error) {
 	return backfill(ctx, w.proc.ds)
 }
 
+// EnqueueStaleAbsentAll requeues known-absent entries older than staleAbsentAge, so artwork
+// that appeared since the last attempt is eventually picked up.
 func (w *Worker) EnqueueStaleAbsentAll(ctx context.Context) error {
 	return enqueueStaleAbsentAll(ctx, w.proc.ds)
 }
 
+// EnqueueMissingAll requeues entities that have no artwork state row at all: the safety net
+// for anything a scan never enqueued.
 func (w *Worker) EnqueueMissingAll(ctx context.Context) error {
 	return enqueueMissingAll(ctx, w.proc.ds)
 }