[SPARK-2259] Fix highly misleading docs on cluster / client deploy modes

The existing docs are highly misleading. For standalone mode, for example, it encourages the user to use standalone-cluster mode, which is not officially supported. The safeguards have been added in Spark submit itself to prevent bad documentation from leading users down the wrong path in the future.

This PR is prompted by countless headaches users of Spark have run into on the mailing list.
Some common deployment strategies are:
1. Run your application in "client mode" from a gateway machine it is physically co-located with your Spark cluster.
2. Submit your application from a machine that is not co-located with the Spark cluster (e.g. a personal laptop)  and run the application in "cluster mode".