ax(mining): replace Manager prose comment with usage example

AX Principle 2: comments must show HOW with real values, not restate
what the type signature already says. Replaced the prose description
on Manager with a concrete call sequence.

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

pkg/mining/manager.go

@@ -28,7 +28,9 @@ type ManagerInterface interface {
 	Stop()
 }
 
-// Manager handles the lifecycle and operations of multiple miners.
+// manager := mining.NewManager()
+// defer manager.Stop()
+// miner, err := manager.StartMiner(ctx, "xmrig", &mining.Config{Algo: "rx/0"})
 type Manager struct {
 	miners      map[string]Miner
 	mu          sync.RWMutex