[SPARK-4864] Add documentation to Netty-based configs

Author: Aaron Davidson <aaron@databricks.com>

Closes #3713 from aarondav/netty-configs and squashes the following commits:

8a8b373 [Aaron Davidson] Address Patrick's comments
3b1f84e [Aaron Davidson] [SPARK-4864] Add documentation to Netty-based configs

(cherry picked from commit fbca6b6ce293b1997b40abeb9ab77b8a969a5fc9)
Signed-off-by: Patrick Wendell <pwendell@gmail.com>