MAPREDUCE-7062. Update mapreduce.job.tags description for making use for ATSv2 purpose. Contributed by Charan Hebri.

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml

@@ -1045,6 +1045,12 @@
     <value></value>
     <description> Tags for the job that will be passed to YARN at submission
       time. Queries to YARN for applications can filter on these tags.
+      If these tags are intended to be used with The YARN Timeline Service v.2,
+      prefix them with the appropriate tag names for flow name, flow version and
+      flow run id. Example:
+      timeline_flow_name_tag:foo,
+      timeline_flow_version_tag:3df8b0d6100530080d2e0decf9e528e57c42a90a,
+      timeline_flow_run_id_tag:1465246348599
     </description>
   </property>