[SPARK-2930] clarify docs on using webhdfs with spark.yarn.access.nam…

…enodes

Author: Tom Graves <tgraves@yahoo-inc.com>

Closes #10699 from tgravescs/SPARK-2930.

docs/running-on-yarn.md

@@ -260,10 +260,10 @@ If you need a reference to the proper location to put log files in the YARN so t
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     A comma-separated list of secure HDFS namenodes your Spark application is going to access. For
-    example, <code>spark.yarn.access.namenodes=hdfs://nn1.com:8032,hdfs://nn2.com:8032</code>.
-    The Spark application must have access to the namenodes listed and Kerberos must
-    be properly configured to be able to access them (either in the same realm or in
-    a trusted realm). Spark acquires security tokens for each of the namenodes so that
+    example, <code>spark.yarn.access.namenodes=hdfs://nn1.com:8032,hdfs://nn2.com:8032,
+    webhdfs://nn3.com:50070</code>. The Spark application must have access to the namenodes listed 
+    and Kerberos must be properly configured to be able to access them (either in the same realm 
+    or in a trusted realm). Spark acquires security tokens for each of the namenodes so that
     the Spark application can access those remote HDFS clusters.
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