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

AX Principle 2 — comments must show HOW with real values, not restate
what the signature already says.

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

pkg/mining/manager.go

@@ -229,8 +229,7 @@ func findAvailablePort() (int, error) {
 	return listener.Addr().(*net.TCPAddr).Port, nil
 }
 
-// StartMiner starts a new miner and saves its configuration.
-// The context can be used to cancel the operation.
+// miner, err := manager.StartMiner(ctx, "xmrig", &mining.Config{Algo: "rx/0"})
 func (m *Manager) StartMiner(ctx context.Context, minerType string, config *Config) (Miner, error) {
 	// Check for cancellation before acquiring lock
 	select {