update documentation for preserve client IP attribute (#1798)

Author: kishorj

docs/guide/service/annotations.md

@@ -105,3 +105,7 @@ for proxy protocol v2 configuration.
             ```
             service.beta.kubernetes.io/aws-load-balancer-target-group-attributes: deregistration_delay.connection_termination.enabled=true
             ```
+        - enable [client IP preservation](https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#client-ip-preservation)
+            ```
+            service.beta.kubernetes.io/aws-load-balancer-target-group-attributes: preserve_client_ip.enabled=true
+            ```