[Docs] Fix outdated docs for standalone cluster

This is now supported!

Author: andrewor14 <andrewor14@gmail.com>
Author: Andrew Or <andrewor14@gmail.com>

Closes #2461 from andrewor14/document-standalone-cluster and squashes the following commits:

85c8b9e [andrewor14] Wording change per Patrick
35e30ee [Andrew Or] Fix outdated docs for standalone cluster

docs/spark-standalone.md

@@ -248,8 +248,10 @@ You can also pass an option `--cores <numCores>` to control the number of cores
 
 The [`spark-submit` script](submitting-applications.html) provides the most straightforward way to
 submit a compiled Spark application to the cluster. For standalone clusters, Spark currently
-only supports deploying the driver inside the client process that is submitting the application
-(`client` deploy mode).
+supports two deploy modes. In `client` mode, the driver is launched in the same process as the
+client that submits the application. In `cluster` mode, however, the driver is launched from one
+of the Worker processes inside the cluster, and the client process exits as soon as it fulfills
+its responsibility of submitting the application without waiting for the application to finish.
 
 If your application is launched through Spark submit, then the application jar is automatically
 distributed to all worker nodes. For any additional jars that your application depends on, you