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

AX Principle 2: comments show HOW with real values, not WHAT the
signature already says.

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

pkg/mining/circuit_breaker.go

@@ -88,7 +88,7 @@ func (cb *CircuitBreaker) State() CircuitState {
 	return cb.state
 }
 
-// Execute runs the given function with circuit breaker protection
+// result, err := cb.Execute(func() (interface{}, error) { return fetchStats(ctx) })
 func (cb *CircuitBreaker) Execute(fn func() (interface{}, error)) (interface{}, error) {
 	// Check if we should allow this request
 	if !cb.allowRequest() {