Add comment to specify default

pkg/apis/core/types.go

@@ -2599,6 +2599,7 @@ type PodSpec struct {
 	RuntimeClassName *string
 	// EnableServiceLinks indicates whether information about services should be injected into pod's
 	// environment variables, matching the syntax of Docker links.
+	// If not specified, the default is true.
 	// +optional
 	EnableServiceLinks *bool
 }