document requirements for decoders (+ others) specified via class in …

…the config

These requirements arise from the behavior of `.FeignClientFactoryBean.getOrInstantiate`

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

@@ -199,7 +199,7 @@ feign:
 				queryMapEncoder: com.example.SimpleQueryMapEncoder
 				metrics.enabled: false
 ----
-`feignName` in this example refers to `@FeignClient` `value`, that is also aliased with `@FeignClient` `name` and `@FeignClient` `contextId`. In a load-balanced scenario, it also corresponds to the `serviceId` of the server app that will be used to retrieve the instances.
+`feignName` in this example refers to `@FeignClient` `value`, that is also aliased with `@FeignClient` `name` and `@FeignClient` `contextId`. In a load-balanced scenario, it also corresponds to the `serviceId` of the server app that will be used to retrieve the instances. The specified classes for decoders, retryer and other ones must have a bean in the Spring context or have a default constructor.
 
 
 Default configurations can be specified in the `@EnableFeignClients` attribute `defaultConfiguration` in a similar manner as described above. The difference is that this configuration will apply to _all_ feign clients.