[SPARK-1191][RESUBMIT]Missing document of some parameters in Spark Core

It's a resubmission of https://github.com/apache/spark/pull/85 according to @mateiz 's suggestion

----

Hi, @mateiz and @pwendell 

I found that nearly all parameters in constructing actor system (AkkaUtil) are not documented...

IIRC, there was a PR which proposes that we should leave space for the user to configure the Akka system, is that still valid?