Soften security warning and keep it in cluster management docs only, not in the main doc page, where it's not necessarily relevant.

Author: srowen

docs/index.md

@@ -25,11 +25,6 @@ It provides high-level APIs in Java, Scala, Python and R,
 and an optimized engine that supports general execution graphs.
 It also supports a rich set of higher-level tools including [Spark SQL](sql-programming-guide.html) for SQL and structured data processing, [MLlib](ml-guide.html) for machine learning, [GraphX](graphx-programming-guide.html) for graph processing, and [Structured Streaming](structured-streaming-programming-guide.html) for incremental computation and stream processing.
 
-# Security
-
-Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
-Please see [Spark Security](security.html) before downloading and running Spark.
-
 # Downloading
 
 Get Spark from the [downloads page](https://spark.apache.org/downloads.html) of the project website. This documentation is for Spark version {{site.SPARK_VERSION}}. Spark uses Hadoop's client libraries for HDFS and YARN. Downloads are pre-packaged for a handful of popular Hadoop versions.

docs/running-on-kubernetes.md

@@ -25,8 +25,10 @@ Kubernetes scheduler that has been added to Spark.
 
 # Security
 
-Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
-Please see [Spark Security](security.html) and the specific advice below before running Spark.
+Security features like authentication are not enabled by default. When deploying a cluster that is open to the internet
+or an untrusted network, it's important to secure access to the cluster to prevent unauthorized applications
+from running on the cluster.
+Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
 
 ## User Identity
 

docs/running-on-mesos.md

@@ -32,7 +32,9 @@ The advantages of deploying Spark with Mesos include:
 
 # Security
 
-Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Security features like authentication are not enabled by default. When deploying a cluster that is open to the internet
+or an untrusted network, it's important to secure access to the cluster to prevent unauthorized applications
+from running on the cluster.
 Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
 
 # How it Works

docs/running-on-yarn.md

@@ -26,7 +26,9 @@ was added to Spark in version 0.6.0, and improved in subsequent releases.
 
 # Security
 
-Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Security features like authentication are not enabled by default. When deploying a cluster that is open to the internet
+or an untrusted network, it's important to secure access to the cluster to prevent unauthorized applications
+from running on the cluster.
 Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
 
 # Launching Spark on YARN

docs/spark-standalone.md

@@ -25,7 +25,9 @@ In addition to running on the Mesos or YARN cluster managers, Spark also provide
 
 # Security
 
-Security in Spark is OFF by default. This could mean you are vulnerable to attack by default.
+Security features like authentication are not enabled by default. When deploying a cluster that is open to the internet
+or an untrusted network, it's important to secure access to the cluster to prevent unauthorized applications
+from running on the cluster.
 Please see [Spark Security](security.html) and the specific security sections in this doc before running Spark.
 
 # Installing Spark Standalone to a Cluster