docs: Clarify only netty does client side load balancing (#2087)

Author: johanandren

runtime/src/main/resources/reference.conf

@@ -35,7 +35,8 @@ akka.grpc.client."*" {
   port = 0
 
   # Experimental in grpc-java https://github.com/grpc/grpc-java/issues/1771
-  # pick_first or round_robin
+  # Only supported for netty client backend, when using akka-http this setting is ignored.
+  # Valid values are "pick_first" or "round_robin"
   load-balancing-policy = ""
 
   deadline = infinite