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

AX principle 2: comments show HOW with real values, not WHAT the
type already says in its name.

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

pkg/mining/circuit_breaker.go

@@ -49,7 +49,8 @@ func DefaultCircuitBreakerConfig() CircuitBreakerConfig {
 	}
 }
 
-// CircuitBreaker implements the circuit breaker pattern
+// cb := NewCircuitBreaker("github-api", DefaultCircuitBreakerConfig())
+// result, err := cb.Execute(func() (interface{}, error) { return fetchStats(ctx) })
 type CircuitBreaker struct {
 	name           string
 	config         CircuitBreakerConfig