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

AX Principle 2 — comments must show HOW with real values, not describe
WHAT the signature already states.

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

pkg/mining/manager.go

@@ -490,8 +490,8 @@ func (m *Manager) ListMiners() []Miner {
 	return miners
 }
 
-// RegisterMiner registers an already-started miner with the manager.
-// This is useful for simulated miners or externally managed miners.
+// sim := NewSimulatedMiner(SimulatedMinerConfig{Name: "sim-rx0"})
+// if err := manager.RegisterMiner(sim); err != nil { return err }
 func (m *Manager) RegisterMiner(miner Miner) error {
 	name := miner.GetName()