Document new service.agent configuration option. (#753)

docs/api/options.md

@@ -72,6 +72,7 @@
     - `service.rewriteHeaders`: `Function` A function that gets the original headers as a parameter and returns an object containing values that should be added to the headers
     - `service.initHeaders`: `Function` or `Object` An object or a function that returns the headers sent to the service for the initial \_service SDL query.
     - `service.connections`: The number of clients to create. (Default: `10`)
+    - `service.agent`: An optional, fully configured [undici](https://github.com/nodejs/undici) agent/pool instance to use to perform network requests. If used, you must set all connections options on the instance as the request related options from the `service` configuration will not be applied.
     - `service.bodyTimeout`: The timeout after which a request will time out, in milliseconds. (Default: `30e3` - 30 seconds)
     - `service.headersTimeout`: The amount of time the parser will wait to receive the complete HTTP headers, in milliseconds. (Default: `30e3` - 30 seconds)
     - `service.keepAliveMaxTimeout`: The maximum allowed keepAliveTimeout. (Default: `5e3` - 5 seconds)