add doc for clientVariable

client-java/controller-api/src/main/java/org/evomaster/client/java/controller/api/dto/problem/rpc/RPCActionDto.java

@@ -24,6 +24,23 @@ public class RPCActionDto {
 
     /**
      * a variable referring to client instance
+     *
+     * this info would be used in static init declaration referring
+     * to an actual client instance in the generated test
+     * then later, the variable could be used to process rpc function call
+     *
+     * eg, the variable is foo
+     * public class Test{
+     *     private static Client foo;
+     *     ...
+     *     pubic void test(){
+     *         response = foo.bar(request)
+     *     }
+     * }
+     *
+     * TODO
+     * Note that the current test generation is proceeded in the driver
+     * if we move it to core, this info could be removed
      */
     public String clientVariable;