Update docs to include SASL support for RPC

## What changes were proposed in this pull request?

Update docs to include SASL support for RPC

Evidence: https://github.com/apache/spark/blob/master/core/src/main/scala/org/apache/spark/rpc/netty/NettyRpcEnv.scala#L63

## How was this patch tested?

Docs change only

Author: Michael Gummelt <mgummelt@mesosphere.io>

Closes #14549 from mgummelt/sasl.