Fix gh 491 gh 553 non reactive loadbalancer client (#590)

* Provide non-reactive LB client implemenation to use with RestTemplate.
Fixes gh-491.
Fixes gh-553.

* Add javadoc.

docs/src/main/asciidoc/spring-cloud-commons.adoc

@@ -336,7 +336,7 @@ For instance, Eureka's supported statuses are `UP`, `DOWN`, `OUT_OF_SERVICE`, an
 
 === Spring RestTemplate as a Load Balancer Client
 
-`RestTemplate` can be automatically configured to use ribbon.
+`RestTemplate` can be automatically configured to use a Load-balancer client under the hood.
 To create a load-balanced `RestTemplate`, create a `RestTemplate` `@Bean` and use the `@LoadBalanced` qualifier, as shown in the following example:
 
 [source,java,indent=0]
@@ -369,6 +369,15 @@ The URI needs to use a virtual host name (that is, a service name, not a host na
 The Ribbon client is used to create a full physical address.
 See {githubroot}/spring-cloud-netflix/blob/master/spring-cloud-netflix-ribbon/src/main/java/org/springframework/cloud/netflix/ribbon/RibbonAutoConfiguration.java[RibbonAutoConfiguration] for details of how the `RestTemplate` is set up.
 
+IMPORTANT: In order to use a load-balanced `RestTemplate`, you need to have a load-balancer implementation in your classpath.
+The recommended implementation is `BlockingLoadBalancerClient`
+- add `org.springframework.cloud:spring-cloud-loadbalancer` in order to use it.
+The
+`RibbonLoadBalancerClient` also can be used, but it's now under maintenance and we do not recommend adding it to new projects.
+
+WARNING: If you want to use `BlockingLoadBalancerClient`, make sure you do not have
+`RibbonLoadBalancerClient` in the project classpath, as for backward compatibility reasons, it will be used by default.
+
 === Spring WebClient as a Load Balancer Client
 
 `WebClient` can be automatically configured to use a load-balancer client.