[SPARK-8405] [DOC] Add how to view logs on Web UI when yarn log aggregation is enabled

Some users may not be aware that the logs are available on Web UI even if Yarn log aggregation is enabled. Update the doc to make this clear and what need to be configured.

Author: Carson Wang <carson.wang@intel.com>

Closes #7463 from carsonwang/YarnLogDoc and squashes the following commits:

274c054 [Carson Wang] Minor text fix
74df3a1 [Carson Wang] address comments
5a95046 [Carson Wang] Update the text in the doc
e5775c1 [Carson Wang] Update doc about how to view the logs on Web UI when yarn log aggregation is enabled

docs/running-on-yarn.md

@@ -68,9 +68,9 @@ In YARN terminology, executors and application masters run inside "containers".
 
     yarn logs -applicationId <app ID>
     
-will print out the contents of all log files from all containers from the given application. You can also view the container log files directly in HDFS using the HDFS shell or API. The directory where they are located can be found by looking at your YARN configs (`yarn.nodemanager.remote-app-log-dir` and `yarn.nodemanager.remote-app-log-dir-suffix`).
+will print out the contents of all log files from all containers from the given application. You can also view the container log files directly in HDFS using the HDFS shell or API. The directory where they are located can be found by looking at your YARN configs (`yarn.nodemanager.remote-app-log-dir` and `yarn.nodemanager.remote-app-log-dir-suffix`). The logs are also available on the Spark Web UI under the Executors Tab. You need to have both the Spark history server and the MapReduce history server running and configure `yarn.log.server.url` in `yarn-site.xml` properly. The log URL on the Spark history server UI will redirect you to the MapReduce history server to show the aggregated logs.
 
-When log aggregation isn't turned on, logs are retained locally on each machine under `YARN_APP_LOGS_DIR`, which is usually configured to `/tmp/logs` or `$HADOOP_HOME/logs/userlogs` depending on the Hadoop version and installation. Viewing logs for a container requires going to the host that contains them and looking in this directory.  Subdirectories organize log files by application ID and container ID.
+When log aggregation isn't turned on, logs are retained locally on each machine under `YARN_APP_LOGS_DIR`, which is usually configured to `/tmp/logs` or `$HADOOP_HOME/logs/userlogs` depending on the Hadoop version and installation. Viewing logs for a container requires going to the host that contains them and looking in this directory.  Subdirectories organize log files by application ID and container ID. The logs are also available on the Spark Web UI under the Executors Tab and doesn't require running the MapReduce history server.
 
 To review per-container launch environment, increase `yarn.nodemanager.delete.debug-delay-sec` to a
 large value (e.g. 36000), and then access the application cache through `yarn.nodemanager.local-dirs`