Document how captureHeaders: false can improve performance

On the [Performance Tuning page](https://www.elastic.co/guide/en/apm/agent/nodejs/current/performance-tuning.html), we should document the use of the [`captureHeaders`](https://www.elastic.co/guide/en/apm/agent/nodejs/current/configuration.html#capture-headers) config option as a means to reduce storage cost and a slight improvement in performance due to the lower number of objects created and processed.