Add link to the Dataflow service options page (apache#29068)

* Add link to the Dataflow service options page

* Fix link format

* Small text edit

runners/google-cloud-dataflow-java/src/main/java/org/apache/beam/runners/dataflow/options/DataflowPipelineOptions.java

@@ -112,7 +112,10 @@ public interface DataflowPipelineOptions
    */
   @Description(
       "Service options are set by the user and configure the service. This "
-          + "decouples service side feature availability from the Apache Beam release cycle.")
+          + "decouples service side feature availability from the Apache Beam release cycle. "
+          + "For a list of service options, see "
+          + "https://cloud.google.com/dataflow/docs/reference/service-options "
+          + "in the Dataflow documentation.")
   List<String> getDataflowServiceOptions();
 
   void setDataflowServiceOptions(List<String> options);