HDFS-6806. HDFS Rolling upgrade document should mention the versions available. Contributed by J.Andreina.

(cherry picked from commit 82db3341bfb344f10c4c6cc8eea0d8c19e05956a)

Conflicts:
	hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

hadoop-hdfs-project/hadoop-hdfs/CHANGES.txt

@@ -417,6 +417,9 @@ Release 2.7.0 - UNRELEASED
     HDFS-7411. Change decommission logic to throttle by blocks rather than
     nodes in each interval. (Andrew Wang via cdouglas)
 
+    HDFS-6806. HDFS Rolling upgrade document should mention the versions
+    available. (J.Andreina via aajisaka)
+
   OPTIMIZATIONS
 
     HDFS-7454. Reduce memory footprint for AclEntries in NameNode.

hadoop-hdfs-project/hadoop-hdfs/src/site/xdoc/HdfsRollingUpgrade.xml

@@ -47,7 +47,9 @@
     These two capabilities make it feasible to upgrade HDFS without incurring HDFS downtime.
     In order to upgrade a HDFS cluster without downtime, the cluster must be setup with HA.
   </p>
-
+  <p>
+    Note that rolling upgrade is supported only from Hadoop-2.4.0 onwards.
+  </p>
   <subsection name="Upgrade without Downtime" id="UpgradeWithoutDowntime">
   <p>
     In a HA cluster, there are two or more <em>NameNodes (NNs)</em>, many <em>DataNodes (DNs)</em>,