doc: add document on WRR configure examples from service config and xds (#11344)

xds/src/main/java/io/grpc/xds/WeightedRoundRobinLoadBalancer.java

@@ -64,6 +64,30 @@ import java.util.logging.Logger;
  * A {@link LoadBalancer} that provides weighted-round-robin load-balancing over the
  * {@link EquivalentAddressGroup}s from the {@link NameResolver}. The subchannel weights are
  * determined by backend metrics using ORCA.
+ * To use WRR, users may configure through channel serviceConfig. Example config:
+ * <pre> {@code
+ *       String wrrConfig = "{\"loadBalancingConfig\":" +
+ *           "[{\"weighted_round_robin\":{\"enableOobLoadReport\":true, " +
+ *           "\"blackoutPeriod\":\"10s\"," +
+ *           "\"oobReportingPeriod\":\"10s\"," +
+ *           "\"weightExpirationPeriod\":\"180s\"," +
+ *           "\"errorUtilizationPenalty\":\"1.0\"," +
+ *           "\"weightUpdatePeriod\":\"1s\"}}]}";
+ *        serviceConfig = (Map<String, ?>) JsonParser.parse(wrrConfig);
+ *        channel = ManagedChannelBuilder.forTarget("test:///lb.test.grpc.io")
+ *            .defaultServiceConfig(serviceConfig)
+ *            .build();
+ *  }
+ *  </pre>
+ *  Users may also configure through xDS control plane via custom lb policy. But that is much more
+ *  complex to set up. Example config:
+ *  <pre>
+ *  localityLbPolicies:
+ *   - customPolicy:
+ *       name: weighted_round_robin
+ *       data: '{ "enableOobLoadReport": true }'
+ *  </pre>
+ *  See related documentation: https://cloud.google.com/service-mesh/legacy/load-balancing-apis/proxyless-configure-advanced-traffic-management#custom-lb-config
  */
 @ExperimentalApi("https://github.com/grpc/grpc-java/issues/9885")
 final class WeightedRoundRobinLoadBalancer extends RoundRobinLoadBalancer {