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

AX principle 2: comments show HOW with real values, not what the
signature already says. Replace "creates a new miner manager" prose
with a concrete call-site example showing manager lifecycle.

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

pkg/mining/manager.go

@@ -62,7 +62,8 @@ func (m *Manager) emitEvent(eventType EventType, data interface{}) {
 
 var _ ManagerInterface = (*Manager)(nil)
 
-// NewManager creates a new miner manager and autostarts miners based on config.
+// manager := mining.NewManager()
+// defer manager.Stop()
 func NewManager() *Manager {
 	m := &Manager{
 		miners:    make(map[string]Miner),