ax(mining): replace prose comment on NewTaskSupervisor with usage example

AX Principle 2: comments show HOW with real values, not WHAT the
function name already says. All other constructors in the file already
use usage-example comments; NewTaskSupervisor was the outlier.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/supervisor.go

@@ -34,7 +34,9 @@ type TaskSupervisor struct {
 	started bool
 }
 
-// NewTaskSupervisor creates a new task supervisor.
+// supervisor := NewTaskSupervisor()
+// supervisor.RegisterTask("stats-collector", collectStats, 5*time.Second, -1)
+// supervisor.Start()
 func NewTaskSupervisor() *TaskSupervisor {
 	ctx, cancel := context.WithCancel(context.Background())
 	return &TaskSupervisor{