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

AX principle 2: comments show HOW with real values, not what the
signature already says. The old comment restated the function name.

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

pkg/mining/container.go

@@ -27,7 +27,9 @@ type ContainerConfig struct {
 	SimulationMode bool
 }
 
-// DefaultContainerConfig returns sensible defaults for the container.
+// cfg := mining.DefaultContainerConfig()
+// cfg.ListenAddr = ":8080"
+// c := NewContainer(cfg)
 func DefaultContainerConfig() ContainerConfig {
 	return ContainerConfig{
 		Database: database.Config{